User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:56.0) Gecko/20100101 Firefox/56.0 Build ID: 20171024165158 Steps to reproduce: Assume I setup an IMAP account with the following settings in section "Synchronization & Storage": Keep messages for this account on this computer = YES Disk Space / Synchronize all messages locally regardless of age = YES This triggers the download of all remote messages (bodies). The Inbox file size is e.g. 2GB Now I change the settings to: Synchronize the most recent 7 days = YES The click OK, exit and restart Thunderbird Actual results: The Inbox file size remains at 2 GB Expected results: All LOCAL (!) messages (bodies) inside the Inbox file but outside the 7 days time frame should be removed automatically (BUT KEPT on remote IMAP server). The Inbox file size should shrink significantly. But it doesn't. This is a bug.

Gene, please comment. I don't think it will remove message once downloaded when you change the sync time interval. Maybe compacting or repairing the folder would get rid of them. Whether that's a bug depends on how the behaviour is specified. In absence of a detailed spec, I'd call this an enhancement, like "Improve local IMAP sync storage when sync time frame is changed".

You're right, if you set the sync time low, e.g., 3 days, and compact the folders via right-click menu the sizes go way down. For example INBOX folder went from 4.9M to 720b in a test account. Repairing has the same effect but basically deletes the folder file and corresponding .msf file and re-downloads/builds them, so compacting is maybe safer (since files are deleted and re-downloaded). Ideally this should be automatic. However, I think the general tb policy is to not delete locally stored emails spontaneously. If this is good or bad I don't really know. I do see spontaneous delete when a folders is un-subscribed. When re-subscribed it also downloads however it may leave backup files (.e.g., -1, -2, etc) so a space saving may not be fully realized. So compacting still seem like a better work-around.