User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:67.0) Gecko/20100101 Firefox/67.0

Steps to reproduce:

Windows 10
TB version 60.7.2
Gmail Imap account.
Previously downloaded full emails on most folders.
In 'Account Settings' > 'Synchronisation & Storage'
previously and still select: checkbox for Keep messages in all folders for this account
under 'disc Space' section
Stop using 'synchronise all messages locally regardless of age'
select 'Synchronise the most recent 30 days'
click on OK

Exit Thunderbird
Access Profile folders ImapMail folder
deleted the imap.gmail.com.msf file
Accessed imap.gmail.com folder
deleted everything except for filterlog.html and msgFilterRules.dat

Restarted Thunderbird

Actual results:

All gmail files recreated and all emails in all folders synchronised and downloaded.

Expected results:

Expected files to be recreated for all folders, but I only expected the last 30 days worth of emails to be synchronised and downloaded.

All imap gmail folders are set to use account settings.

I tested all the above out after locating a Support Forum question where the person could not get the synchronise 30 days to work on an imap hotmail/outlook account.
https://support.mozilla.org/en-US/questions/1263602
So, having never used that setting I thought I'd check it out for my imap gmail account and do not seem able to get an expected result either.

Also tested
In 'Account Settings' > 'Synchronisation & Storage'
uncheck checkbox for Keep messages in all folders for this account
under 'disc Space' section
Stop using 'synchronise all messages locally regardless of age'
select 'Synchronise the most recent 30 days'
click on OK
got a pop up saying I had made chnages to synchronising setting and I selected ok

Exit Thunderbird
Access Profile folders ImapMail folder
deleted the imap.gmail.com.msf file
Accessed imap.gmail.com folder
deleted everything except for filterlog.html and msgFilterRules.dat

Restarted Thunderbird

All folders recreated and synchronised all emails irrespective of date. This time only headers, but it still failed to only synch 30 days.

Gmail support is not our forte.

I don't see that this is gmail specific but, instead, a general imap issue.

I never use this feature and have never dealt with problem about this, so I just now tried it on a gmail account. What I see is that when you set the parameter to sync only X days, restart tb to make sure the change takes effect and then "repair" gmail's INBOX (right click properties on INBOX folder), the INBOX file does now only contain the emails from the last X days. But you still see all your INBOX emails in tb. I was sort of expecting to only see the emails from X days but instead I still see them all. I'm not sure if this is what the reporter means when "failed to only synch 30 days" is stated.

I did notice that this feature is associated with the autosync feature. I had autosync disabled for other reasons when I first tried testing this. With autosync off, after a repair the INBOX file is completely empty.

Another thing I notice is that, with autosync enabled and after the repair, initally only the messages from X days are contain in the INBOX file. However, when I click and read a message older than X days, it is added to INBOX file. I'm not sure if this is a bug or feature.

Also, I'm not sure what happens when time moves on. Do the mails in the INBOX file that are older than X days, including old emails I have recently click on and read, get purged from the INBOX file? I would hope that autosync does this, but not sure.

...Well, doing some tests, I don't see autosync purging the old messages. However, I do see that when I right-click "compact" the INBOX folder, the older recently read messages are removed from the file (but are still accessible in tb).

Bug 510707 adds the UI to support the feature and references bug 482476 where the feature was added. However, I can't find any detailed and definitive documentation on the "Synchronize the most recent X days" feature. Reading the bugs, when older messages are read they are stored in INBOX file (or whatever folder's file) and marked for deletion. It appears that the delete from the file (not the server!) occurs when the compaction threshold is reached (by default, user is prompted to allow the compaction in all folders of all accounts) or when the user selects the folder and compacts just that folder.

I'm not sure if over time messages in the file that become older than the "X days" threshold are marked for deletion in the file or not. This seem like it should occur as part of the background "autosync" process but I haven't verified that it works or not.
Edit: P/S: I have set up a test with my "bugzilla" folder and set it to keep 2 days. I will see tomorrow if the now 2 day old emails get removed from the file somehow.

This issue was originally mentioned for a hotmail/outlook account, so I tested on my gmail account. Both imap accounts displayed same behaviour in that all good emails in all folders were downloaded, so it is not gmail specific.
This issue has nothing to do with marked as deleted emails or compacting as all emails displayed are supposed to be in the server Inbox or indeed any server folder.
This issue has nothing to do with whether emails are fully downloaded and stored in mbox files or whether only headers are downloaded and you see only index files in the mail account profile files as it occurs in both situations.
All files contained in the profile folders - whether mbox or index were deleted whilst Thunderbird was closed, so this was a 'clean' synchronisation to 'Synchronise the most recent 30 days'. Repairing index files was not required as they had been deleted.

I would expect on the first occasion for only 30 days to be downloaded. As days progress, I would expect to see the number of emails increase as previously downloaded emails would still be visible assuming nothing is deleted or archived or moved to folders that are not synchronised. I do not see how previously downloaded emails that are not deleted could be erased from mbox file without deleting off server; hence increase in displayed emails, but it should not be displaying emails older than the initial 30 day download. It would require all the mbox and index files to be removed whilst thunderbird was closed to force another clean synch for only 30 days. This helps to keep low numbers in imap account on computer whilst still storing emails on server. It also means at any point, I could change server settings to force a full download for backup purposes.

However, this feature is failing to work.

First of all, this feature does not stop any emails from being seen in tb and it doesn't cause any to be deleted from the server.

Here's how I see it working based on experiments and somewhat verified by reading the code and the original enhancement bug reports: Assume you have a folder with a huge offline store file, e.g., INBOX, and you want to reduce the amount of offline storage on disk. So you change the parameter to 30 days for the account (this will affect all folders in the account and not just INBOX). Then for good measure, restart tb. Now right click on INBOX and repair the folder or, alternately, delete the INBOX and INBOX.msf file with tb shutdown. Either way, INBOX and INBOX.msf file will be rebuilt and you will only see the last 30 days of email in the INBOX file so it should be smaller than it originally was.

With just 30 days of email in the INBOX file you will still see all your older emails in tb but the email bodies won't be downloaded, only the headers will be fetched and the information encoded into the INBOX.msf file.

Now when you access an email older than 30 day, it will be fully downloaded and stored in INBOX. It will be flagged in the .msf with "removal pending" attribute. Also, as time moves on, emails stored in INBOX older than 30 days will also get flagged with the "removal pending" attribute. There is a another parameter in advanced preference that says "Compact all folders when it will save over X MB". This is checked on startup or once per day (I think) while tb is continuously running. When the threshold is reached, there will a prompt to compact your folders. If you compact them, any message previously flagged with "removal pending" will be removed from the file. Messages marked with "removal pending" won't be removed from the server so they will still be visible in tb. This also does not appear to do an imap EXPUNGE on the server's folders.

Alternatively, you can also compact the folder using the right-click menu. This will have a the same effect on the messages with "removal pending" set but it will also do an imap EXPUNGE on the imap folder at the server so any messages in the mailbox (folder) that are marked as \deleted will be permanently deleted. Of course, these are messages you have already deleted so probably this is not a problem.

The only bug I see here is that how this fully works was never really documented that I could find.

(In reply to Anje from comment #5)

This issue was originally mentioned for a hotmail/outlook account, so I tested on my gmail account. Both imap accounts displayed same behaviour in that all good emails in all folders were downloaded, so it is not gmail specific.

I agree.

This issue has nothing to do with marked as deleted emails or compacting as all emails displayed are supposed to be in the server Inbox or indeed any server folder.
This issue has nothing to do with whether emails are fully downloaded and stored in mbox files or whether only headers are downloaded and you see only index files in the mail account profile files as it occurs in both situations.
All files contained in the profile folders - whether mbox or index were deleted whilst Thunderbird was closed, so this was a 'clean' synchronisation to 'Synchronise the most recent 30 days'. Repairing index files was not required as they had been deleted.

Repairing a folder is almost the same as deleting and allow the folder to re-download. You just don't have to manually go into the command line or a file manager to do it. I haven't tested it, but maybe just setting the parameter to 30 days or whatever affects the mbox file (marks older messages with pending removal). If so, manually compacting the folder should bring the size down. Again, I haven't tried this to verify.

I would expect on the first occasion for only 30 days to be downloaded.

All headers will will be downloaded but only 30 days of email bodies will be downloaded. You should have a much smaller mbox file after the download. When you look at the mbox text you should see only dates within the last 30 days. Do you?

As days progress, I would expect to see the number of emails increase as previously downloaded emails would still be visible assuming nothing is deleted or archived or moved to folders that are not synchronised.

That's true, but the emails that become older than 30 days remain in the mbox file but are marked in the .msf file as "pending removal".

I do not see how previously downloaded emails that are not deleted could be erased from mbox file without deleting off server; hence increase in displayed emails, but it should not be displaying emails older than the initial 30 day download. It would require all the mbox and index files to be removed whilst thunderbird was closed to force another clean synch for only 30 days. This helps to keep low numbers in imap account on computer whilst still storing emails on server. It also means at any point, I could change server settings to force a full download for backup purposes.

You could re-do the procedure (delete the files or repair the folder) so you have only 30 days in the file again. But a folder compact will have the same effect since it will remove the older messages from the mbox file marked with "pending removal". It won't remove them from the server but a manual compact will imap expunge the mailbox/folder.

However, this feature is failing to work.

Seems to work for me, at least in the way I have tried to describe it.

re:Here's how I see it working based on experiments and somewhat verified by reading the code and the original enhancement bug reports: Assume you have a folder with a huge offline store file, e.g., INBOX, and you want to reduce the amount of offline storage on disk. So you change the parameter to 30 days for the account (this will affect all folders in the account and not just INBOX). Then for good measure, restart tb. Now right click on INBOX and repair the folder or, alternately, delete the INBOX and INBOX.msf file with tb shutdown. Either way, INBOX and INBOX.msf file will be rebuilt .....
Correct so far...

re :and you will only see the last 30 days of email in the INBOX file so it should be smaller than it originally was.
No. All emails are downloaded regardless of date.

There is no point compacting a folder when it no deleted emails.
Folder was previously compacted and is not relevant to this issue.
I repeat - all emails are good emails and all are visible when only those dated within 30 days should be visible.

re:You could re-do the procedure (delete the files or repair the folder) so you have only 30 days in the file again.
It seems we are not on the page as you do not appear to understand that at no point did the procedure ever only download 30 days.
That is the problem.

(In reply to Anje from comment #7)

re :and you will only see the last 30 days of email in the INBOX file so it should be smaller than it originally was.
No. All emails are downloaded regardless of date.

So the file is not smaller? Also, when you look inside the file at the concatenated emails, you see emails with "Date:" fields older than 30 days?

There is no point compacting a folder when it no deleted emails.

Compacting is the only way the emails marked as "pending removal" will be purged from the mbox file. Only if they are marked with imap \deleted will they be removed from the server.

Folder was previously compacted and is not relevant to this issue.
I repeat - all emails are good emails and all are visible when only those dated within 30 days should be visible.

No, you will still see all your emails with this feature. This was discussed in the referenced bug reports when the feature was added.

re:You could re-do the procedure (delete the files or repair the folder) so you have only 30 days in the file again.
It seems we are not on the page as you do not appear to understand that at no point did the procedure ever only download 30 days.
That is the problem.

Well, maybe I'm missing something (not unusual). But what evidence do you have that it downloaded more than 30 days (other than the fact that you still see all you emails possibly going back years)?

Anje,
I couldn't find the explanation in the bug reports. But this comment, which answers the one above it, explains that you will see all your old emails even if you only "sync" the last 15 days:
http://forums.mozillazine.org/viewtopic.php?f=39&t=1707855&p=8560255#p8560255

The best explanation for the compact/"mark for removal" behavior I could find is here: bug 482476 comment 14. It can also be inferred by reading the code (but tb is not easy code). Edit: FYI, the "offline store" referred to is actually the mbox file.

re: So the file is not smaller?
If only headers for download has been selected then the mbox no longer exists, so in that respect it has got smaller.
So dependent on settings the profile folder mbox file will be smaller or not exist.
eg: mbox size 104Kb and index 54Kb
The 'Layout' > 'Folder Pane Columns' 'Size' column will still tell you it is 2MB, so this has nothing to do with mbox or index in profile folders, but is the size of server Inbox.
However, you get all kinds of changes if you go into offline mode, but not perhaps the one you necessarilly expect.
Go offline and you just might get the whole lot downloaded again.

I believe the following may be an explanation for some cases, but not all.
If I choose to only download headers:
In 'Account Settings' > 'Synchronisation & Storage'
uncheck checkbox : 'Keep messages in all folders for this account on this computer'
Click on 'Advanced' button and make sure no checkboxes are selected.

Then all emails will have the headers downloaded and I will see all emails regardless of date in online mode
So if anyone expected to only see the last 30 days then they are mistaken.
AND these settings.....
'Synchronise all messages locally regardless of age'
OR
'synchronise the most recent 30 days'
although selected...will be completely and totally ignored because synchronise is only refering to the downloading of full emails and no folders were selected under 'Advanced'.

Surely those options should be greyed out if no folders are selected?

If the following is selected then all kinds of weirdness occur:
In 'Account Settings' > 'Synchronisation & Storage'
uncheck checkbox : 'Keep messages in all folders for this account on this computer'
Click on 'Advanced' button and select checkbox for eg: Inbox
So all emails will have the headers downloaded as before and I will see all emails regardless of date
But one folder - Inbox - is now selected and therefore the following options should now be enforced on that folder when going offline.

'Synchronise all messages locally regardless of age'
OR
'synchronise the most recent 30 days' (I've selected this one)
whatever option is selected, it should be honoured when I go offline to synchronise specified amount in specified folder(s).

Except that is not necessarilly the case.............
In offline mode - clicked on 'two blue screen icon' in Status bar...., (Note sometimes I get the 'alert' mentioned later on) I can see any email which I had selected to see when in previous online mode.
So, if I had only checked emails dated today, then only emails dated today will be shown in offline mode.
If I go back to online mode and select an email dated three months old, then in offline mode I will still see them.
So any email in Inbox which was selected to view - regardless of date will be synchronised for offline viewing.
But any email in Inbox which was not selected to view in online mode - regardless of date will be not be synchronised for offline viewing.
Meaning I can see emails dated several months old but not yesterdays - assuming they were not selected to view in online mode.
So simply going offline - synchronise recent 30 days does not occur. I'm still not sure why it has synchronised emails just because I selected to view them.
This does not sound correct - something is amiss.

However, providing at least one folder is selected in Account Settings:
In 'Account Settings' > 'Synchronisation & Storage'
uncheck checkbox : 'Keep messages in all folders for this account on this computer'
Click on 'Advanced' button and select checkbox for eg: Inbox
select 'synchronise the most recent 30 days'
clickon OK

Then Exit Thunderbird and delete all mbox, index etc as before, then restart Thunderbird.
This will create appropriate index files and Inbox mbox file with last 30 days synchronised.
So at least that part worked under those specific procedures and only until you start to use Thunderbird.

Then in online mode select a very old header of email to read and you will find it has been fully downloaded and stored in Inbox mbox file.
Am I completely missing something here?
Why are old emails being added to an mbox file when I have settings to only synchronise emails that are 30 days old, so oldest email allowed to be synchronised in the mbox file should be 17th June 2019.

Emails older than 30 days old will get added to mbox file - synchronised for offline use - if downloaded headers are opened/selected to read, even when settings state 'synchronise only 30 days'.

Can you confirm that the option:
select 'synchronise the most recent 30 days'
Only occurs ONCE when you Exit Thunderbird and delete all profile files and then restart Thunderbird.
And thereafter any selected downloaded header in that folder - older than the initial 30 day will get synchronised and added to mbox file for offline viewing.

The select 'synchronise the most recent 30 days' setting is only effective the once when you initially restart after accessing and deleteing files from profile.

It does not mean nothing older than that initial 30 day date will get synchronised, because it can and does.

OK..I've just gone offline using 'two blue screen icon'
Status bar message, connecting to server...Inbox downloaded all emails.
A pop up occurred - alert The operation failed because another operation is using the folder, please wait for that opration to finish and then try again.
click on OK
Then you are finally offline.
This time instead of only seeing any email previously selected or only seeing the 30 days worth which I've just set up.
You can now read all emails regardless of date.
mbox file has just gone up to 2157KB which is total size on server.
Synchronise 30 days is not being honoured when you go offline.

It cannot be correct to need to Exit Thunderbird and delete everything just to start Thunderbird and force it to synch 30 days.

Edit: Some thing I stated below are wrong. See the next comment 12 for details
Anje, Hopefully it is now clear that if you set an account to sync X days, you still see all your emails, even those older than X days. Also, when you access (read) an email older than X days it gets stored in the mbox file but is marked "pending removal" and it will only be removed when the folder is compacted. So the mbox can still get bigger than it would be with just 30 days of email. This can be managed so you get an auto-compact prompt by setting the advanced parameter Compact all folder when it will save over Y MB in total. Or you can also manually select the large folder and compact it. This won't delete any messages that were not already "IMAP marked for deletion" from the server.

Then you bring up issues with reading messages offline. [Edit: wrong] When you click the "two blue screen" icon and go offline it prompts to download all folders in all accounts or don't download (i.e., Later). If you choose to download, all your emails that are not already downloaded in all accounts and folder will be downloaded. However, the emails that are not configured to be synced in an account (older than X days) will still be downloaded. However, when stored in the mbox file they will (or should) be marked "pending removal". Now you can read all your emails in the folders of all your accounts. Regardless of the "sync X days" setting, for offline use all mail must be downloaded in the folder so it works the same as it did when online.

Alternatively, when you want to go offline and you anticipate that you only need one or two folders to read, you can right-click on the folder(s), select them for offline (checkbox) and download only those folders (button) and then go offline and select "Later" so nothing more is downloaded. In this case, all the emails in just that/those folders(s) will have downloaded but those older than X days are marked "pending removal". (Note: if you fail to tick the Select this folder for offline use checkbox and just click the "Download now" button, the created mbox file seem to not have any old entries marked "pending removal". Seems like a bug.)

In both offline cases, when you go back online and if the total size of messages in mboxes marked "pending removal" exceeds Y in the Compact all folders when it saves over Y MB in total parameter, you will be prompted to compact all the folders. Or you can always manually compact them. In the 2nd case, you should also clear the offline selection for the folders you selected for offline use so the mbox files remain empty.

(In reply to Anje from comment #10)

However, you get all kinds of changes if you go into offline mode, but not perhaps the one you necessarilly expect.
Go offline and you just might get the whole lot downloaded again.

Of course, nothing more is downloaded if you click "Later". If you click OK you will get all [Edit: not all, just folders you have marked for offline storage] your email downloaded so you can see it offline. Seems like that's what you would expect.

I believe the following may be an explanation for some cases, but not all.
If I choose to only download headers:
In 'Account Settings' > 'Synchronisation & Storage'
uncheck checkbox : 'Keep messages in all folders for this account on this computer'
Click on 'Advanced' button and make sure no checkboxes are selected.

Then all emails will have the headers downloaded and I will see all emails regardless of date in online mode
So if anyone expected to only see the last 30 days then they are mistaken.

Right. That's probably not clear to lots of users looking at this feature.

AND these settings.....
'Synchronise all messages locally regardless of age'
OR
'synchronise the most recent 30 days'
although selected...will be completely and totally ignored because synchronise is only refering to the downloading of full emails and no folders were selected under 'Advanced'.

Surely those options should be greyed out if no folders are selected?

Not sure it really matters. When you go offline, tb has to download all the emails regardless of age for them to be read by the user and so it mirrors the folder behavior that is seen when online. [Edit: again, I'm wrong about "all".]

I haven't tried to go offline and download everything on this laptop since it has a very small SSD (chromebook running linux) so I run with no offline store [Edit: not really a problem.] But maybe you could test this [Edit: test not needed]: Set to sync all regardless of age. And have no folder sync'd so no mbox files exist. Then go offline and allow everything to download. Now there should a mbox file for each folder. Then go back offline. Do the mbox files vanish or stick around? Do you get a prompt to compact your folders? (To get the prompt you may have to adjust the parameter Compact all folders when it saves over Y MB in total and restart tb.) After compacting (if prompted) do the mbox files vanish or at least become 0 in size? If no prompt for compact, does manually compacting (right-click) affect the mbox file?

I would hope that if no folder is selected for offline use and you go offline and download them all that they will all be marked with "pending removal" so they can be compacted back to zero when you go back online.

If the following is selected then all kinds of weirdness occur:
In 'Account Settings' > 'Synchronisation & Storage'
uncheck checkbox : 'Keep messages in all folders for this account on this computer'
Click on 'Advanced' button and select checkbox for eg: Inbox
So all emails will have the headers downloaded as before and I will see all emails regardless of date
But one folder - Inbox - is now selected and therefore the following options should now be enforced on that folder when going offline.

'Synchronise all messages locally regardless of age'
OR
'synchronise the most recent 30 days' (I've selected this one)
whatever option is selected, it should be honoured when I go offline to synchronise specified amount in specified folder(s).

Except that is not necessarilly the case.............
In offline mode - clicked on 'two blue screen icon' in Status bar...., (Note sometimes I get the 'alert' mentioned later on) I can see any email which I had selected to see when in previous online mode.

Offline mode should mirror (look just like) online mode so you should see all the emails listed and be able to read the emails when in offline mode (using the now fully download mbox file). [Edit: Just for folders selected for offline storage, not every folder.]

So, if I had only checked emails dated today, then only emails dated today will be shown in offline mode.

You're not seeing emails except today's when offline?

If I go back to online mode and select an email dated three months old, then in offline mode I will still see them.

Sounds like the mbox file is not getting downloaded when you go offline. I see the alert too. Be sure to do the download again after you see the alert. [Edit: I think the alert happens when you try to do something else while the download is in progress. You have to keep your hands off the keyboard until the download completes and tb goes offline.]

So any email in Inbox which was selected to view - regardless of date will be synchronised for offline viewing.
But any email in Inbox which was not selected to view in online mode - regardless of date will be not be synchronised for offline viewing.
Meaning I can see emails dated several months old but not yesterdays - assuming they were not selected to view in online mode.
So simply going offline - synchronise recent 30 days does not occur. I'm still not sure why it has synchronised emails just because I selected to view them.
This does not sound correct - something is amiss

So are you saying you can't read emails when offline unless you had previously accessed/read them while online? This works OK for me when I download a single folder and then go offline (and retry the download when I see the alert).

However, providing at least one folder is selected in Account Settings:
In 'Account Settings' > 'Synchronisation & Storage'
uncheck checkbox : 'Keep messages in all folders for this account on this computer'
Click on 'Advanced' button and select checkbox for eg: Inbox
select 'synchronise the most recent 30 days'
clickon OK

Then Exit Thunderbird and delete all mbox, index etc as before, then restart Thunderbird.
This will create appropriate index files and Inbox mbox file with last 30 days synchronised.
So at least that part worked under those specific procedures and only until you start to use Thunderbird.

Then in online mode select a very old header of email to read and you will find it has been fully downloaded and stored in Inbox mbox file.
Am I completely missing something here?
Why are old emails being added to an mbox file when I have settings to only synchronise emails that are 30 days old, so oldest email allowed to be synchronised in the mbox file should be 17th June 2019.

Emails older than 30 days old will get added to mbox file - synchronised for offline use - if downloaded headers are opened/selected to read, even when settings state 'synchronise only 30 days'.

Yes, that seems to be the way it works. I couldn't find a rationale in the only bug report that documents the design. I suppose it is OK since the assumption is that you are accessing an old email, possibly with big attachments, so it can just be stored back in the mbox file for access efficiency. This can be justified since it is marked "pending removal" and when the space threshold is reached, it can be compacted out if the user wants to. The same occurs automatically for newer emails as times moves on: they get marked "pending removal" but remain in the mbox file until the next compact occurs (either manually by the user right-click or the auto-compact prompt when tb determines the potential empty space threshold has been reached).

Can you confirm that the option:
select 'synchronise the most recent 30 days'
Only occurs ONCE when you Exit Thunderbird and delete all profile files and then restart Thunderbird.
And thereafter any selected downloaded header in that folder - older than the initial 30 day will get synchronised and added to mbox file for offline viewing.

You can also just repair the folder. It will have almost the same effect as deleting the files. Yes, if you sync X days and you look at a message older than X days, it will be added to the mbox file and marked "pending removal". That's how it works.

The select 'synchronise the most recent 30 days' setting is only effective the once when you initially restart after accessing and deleteing files from profile.

It does not mean nothing older than that initial 30 day date will get synchronised, because it can and does.

Yes, and it's a feature and now a bug (I think) :).

OK..I've just gone offline using 'two blue screen icon'
Status bar message, connecting to server...Inbox downloaded all emails.
A pop up occurred - alert The operation failed because another operation is using the folder, please wait for that opration to finish and then try again.
click on OK

Not sure what this alert is about. But if you see it you have to retry the download (or attempt to go offline) so the mbox files are fully downloaded via imap fetches.

Then you are finally offline.
This time instead of only seeing any email previously selected or only seeing the 30 days worth which I've just set up.
You can now read all emails regardless of date.
mbox file has just gone up to 2157KB which is total size on server.
Synchronise 30 days is not being honoured when you go offline.

It cannot be correct to need to Exit Thunderbird and delete everything just to start Thunderbird and force it to synch 30 days.

Repair of folder should work too. The mbox file being large is exactly what you want when offline. Otherwise how can you read old emails? The 30 day limit is n/a when you are offline. However, in the mbox file the messages older than 30 days should still be marked "pending removal" so when you go back on line you should see a prompt to compact all folders or right-click compact of Inbox should bring the mbox size back down. It works for me for a single folder for offline use but I can't now test it for all folders downloaded for offline due to small disk size. [Edit: I did the test on dev system as described in comment 12 below.]

Ok, sorry, like I said, I rarely (actually never) use the offline state of tb so I am wrong about how it works. I thought when you click the "double blue screens" that it would download all your folders in all your accounts. I see now that when I try it on my dev system (not the chromebook but a "real" computer with a hard drive) that only the folders that I have marked for offline use are actually downloaded. It scans though all your accounts and downloads only the folders already selected for offline storage. Doing this on the chromebook with minimal disk doesn't cause everything to be downloaded like I feared since I only have a few small folder set for offline storage mainly for test purposes.

Anyhow, I had asked you to test this but that won't be necessary since I see how it works now. I set the accounts to sync 1 year back (365 days) and when the download finishes you can access all the messages in only the "offline" folders when offline, not just those less than 1 year old. The mbox files are also much larger after the download. But when you go back online and restart tb, I see the "compact all folders" prompt and when I do the compact, the mbox files go back to their smaller original size. So this works like I expected; the older messages downloaded before going offline are marked "pending removal" so they are compressed out of the mbox file when a compact operation occurs (either automatically or manually).

re:But when you go back online and restart tb, I see the "compact all folders" prompt and when I do the compact, the mbox files go back to their smaller original size.

I do not get the auto prompt, probably due to threshhold not being reached, but I did a manual compact and agree with your comment that size is reduced again in the mbox file.

I agree it works in a fashion, but is perhaps not intuitive to a general user.
It's not a process I've used before, but having received Support Questions I needed to find out what was going on before offering advice. With your help I've managed to understand exactly how this function works, what to expect and how to maintain it. Cheers.

I do not get the auto prompt, probably due to threshold not being reached, but I did a manual compact and agree with your comment that size is reduced again in the mbox file.

I had a lot of emails with big attachments that were old so they more than reached my auto-compact threshold when back online. I agree that if you set the threshold lower you would also see the prompt.

Yes, not much about this is intuitive. I would like to fix it so that it is but I'm afraid I really don't know what to change and, like most things in tb, it would be controversial. I think a reasonable solution would be to document exactly how this works somewhere that users can access (other than just this bug report).
So, unless someone objects, I will close this bug as invalid.

re: I think a reasonable solution would be to document exactly how this works somewhere that users can access (other than just this bug report).
So, unless someone objects, I will close this bug as invalid.

Agree with you.