User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.131 Safari/537.36 Edg/92.0.902.73

Steps to reproduce:

Several customers report not being able to send mail from Thunderbird. These are all customers using a mailaccount from the dutch provider Zeelandnet. I do not now if it happens at other providers as well, but I haven't seen this yet. I tried updating 91 to 91.0.1, this didn't help. Rolling back to 78.9.1 did solve the problem. Also other mail apps like Outlook and the Windows mail app do work on the same machines with exactly the same settings. This is a SMTP connection, secured with SSL and a normal password, working on port 465.

Actual results:

When trying to send an email, an error appears:
Verzenden van het bericht is mislukt.
Het bericht kan niet worden verzonden via uitgaande (SMTP-)server mail.zeelandnet.nl om een onbekende reden. Controleer of de instellingen van uw uitgaande (SMTP-)server juist zijn en probeer het opnieuw.

Expected results:

The mail should have been sent without an error.

Thanks for reporting. Can you provide some debug logs by following

1. open Config Editor, set mailnews.smtp.loglevel to All
2. open DevTools, go to the Console tab, clear any existing logs
3. send a mail
4. copy the logs from DevTools to here

All on Windows with the same antivirus software? Try to disable the antivirus and send again.

(In reply to Ping Chen (:rnons) from comment #1)

Thanks for reporting. Can you provide some debug logs by following

1. open Config Editor, set mailnews.smtp.loglevel to All
2. open DevTools, go to the Console tab, clear any existing logs
3. send a mail
4. copy the logs from DevTools to here

This happens on customer's PC's I do not currently have access to. I will try if I can replicate the issue on a PC of ours and let you know.

(In reply to Richard Marti (:Paenglab) from comment #2)

All on Windows with the same antivirus software? Try to disable the antivirus and send again.

Yes, this happens on Windows PC's with Kaspersky. Unfortunately disabling Kaspersky doesn't solve the issue. Installing the older version of Thunderbird does solve the issue. Even with Kaspersky turned on.

Kaspersky doesn't disable totally. You should check if Kaspersky does traffic inspection. It's very possible Kaspersky doesn't know the new TB and blocks it. On every new big release we see this problems with different antivirus tools.

(In reply to Richard Marti (:Paenglab) from comment #5)

Kaspersky doesn't disable totally. You should check if Kaspersky does traffic inspection. It's very possible Kaspersky doesn't know the new TB and blocks it. On every new big release we see this problems with different antivirus tools.

You are right, uninstalling Kaspersky is often better for solving this kind of issues than just turning it off. Unfortunately not in this case. I just installed Thunderbird 91 on a notebook of mine, on a clean Windows 10 without any Antivirus. When I set up a Zeelandnet-mail account, I get the same error.

(In reply to Ping Chen (:rnons) from comment #1)

Thanks for reporting. Can you provide some debug logs by following

1. open Config Editor, set mailnews.smtp.loglevel to All
2. open DevTools, go to the Console tab, clear any existing logs
3. send a mail
4. copy the logs from DevTools to here

These are the logs from DevTools:

Gebruik van window.controllers/Controllers wordt niet meer ondersteund. Gebruik dit niet voor UA-detectie. blank
[l10nregistry] Attempting to synchronously load file resource:///chrome/en-US/locale/en-US/calendar/toolkit/intl/languageNames.ftl while it's being loaded asynchronously. L10nRegistry.jsm:597:19
[Exception... "Component returned failure code: 0x80520001 (NS_ERROR_FILE_UNRECOGNIZED_PATH) [nsIXPCComponents_Utils.readUTF8URI]" nsresult: "0x80520001 (NS_ERROR_FILE_UNRECOGNIZED_PATH)" location: "JS frame :: resource://gre/modules/L10nRegistry.jsm :: L10nRegistry.loadSync :: line 692" data: no] L10nRegistry.jsm:692:19
    loadSync resource://gre/modules/L10nRegistry.jsm:692
    fetchFile resource://gre/modules/L10nRegistry.jsm:607
    generateResourceSetSync resource://gre/modules/L10nRegistry.jsm:512
    map self-hosted:221
    generateResourceSetSync resource://gre/modules/L10nRegistry.jsm:507
    generateResourceSetsForLocaleSync resource://gre/modules/L10nRegistry.jsm:449
    next self-hosted:1430
    generateBundlesSync resource://gre/modules/L10nRegistry.jsm:186
    next self-hosted:1430
    touchNext resource://gre/modules/Localization.jsm:167
    generateBundles resource://gre/modules/Localization.jsm:473
    getLanguageDisplayNames resource://gre/modules/mozIntl.jsm:788
    getLocaleDisplayNames resource://gre/modules/mozIntl.jsm:866
    map self-hosted:221
    getLocaleDisplayNames resource://gre/modules/mozIntl.jsm:845
    sortDictionaryList resource://gre/modules/InlineSpellChecker.jsm:203
    InitLanguageMenu chrome://messenger/content/messengercompose/MsgComposeCommands.js:5764
    updateLanguageInStatusBar chrome://messenger/content/messengercompose/MsgComposeCommands.js:5848
    InterpretGeneratorResume self-hosted:1482
    AsyncFunctionNext self-hosted:692
    (Async: async)
    gLanguageObserver chrome://messenger/content/messengercompose/MsgComposeCommands.js:3293
    forEach self-hosted:205
    gLanguageObserver chrome://messenger/content/messengercompose/MsgComposeCommands.js:3291
    (Async: MutationCallback)
    ComposeStartup chrome://messenger/content/messengercompose/MsgComposeCommands.js:3290
    ComposeLoad chrome://messenger/content/messengercompose/MsgComposeCommands.js:4061
    <anoniem> chrome://messenger/content/messengercompose/MsgComposeCommands.js:211
    (Async: EventListener.handleEvent)
    <anoniem> chrome://messenger/content/messengercompose/MsgComposeCommands.js:210
Prompter: internal dialogs not available in this context. Falling back to window prompt. Prompter.jsm:1084
    set modalType resource://gre/modules/Prompter.jsm:1084
    ModalPrompter resource://gre/modules/Prompter.jsm:1040
    getPrompt resource://gre/modules/Prompter.jsm:65
    CompleteGenericSendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:4874
    GenericSendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:4810
    SendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:5329
    doCommand chrome://messenger/content/messengercompose/MsgComposeCommands.js:948
    doCommand chrome://messenger/content/messengercompose/MsgComposeCommands.js:1126
    goDoCommand chrome://global/content/globalOverlay.js:123
    oncommand chrome://messenger/content/messengercompose/messengercompose.xhtml:1
Invalid Console.maxLogLevelPref value: all 22
mailnews.smtp: Command failed: 501 <username-removed@zeelandnet.nl> BODY=8BITMIME SMTPUTF8: malformed address: BODY=8BITMIME SMTPUTF8 may not follow <username-removed@zeelandnet.nl> ; currentAction=_actionStream SmtpClient.jsm:508:19
    _onCommand resource:///modules/SmtpClient.jsm:508
    _parse resource:///modules/SmtpClient.jsm:370
    _onData resource:///modules/SmtpClient.jsm:418
    _onOpen resource:///modules/SmtpClient.jsm:397
    connect resource:///modules/SmtpClient.jsm:152
    sendMailMessage resource:///modules/SmtpService.jsm:92
    _deliverFileAsMail resource:///modules/MessageSend.jsm:1039
    _deliverMessage resource:///modules/MessageSend.jsm:757
    createAndSendMessage resource:///modules/MessageSend.jsm:138
    CompleteGenericSendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:4874
    GenericSendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:4810
    SendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:5329
    doCommand chrome://messenger/content/messengercompose/MsgComposeCommands.js:948
    doCommand chrome://messenger/content/messengercompose/MsgComposeCommands.js:1126
    goDoCommand chrome://global/content/globalOverlay.js:123
    oncommand chrome://messenger/content/messengercompose/messengercompose.xhtml:1
Invalid Console.maxLogLevelPref value: all
mailnews.smtp: Message sending failed. SmtpClient.jsm:1300:21
    _actionStream resource:///modules/SmtpClient.jsm:1300
    _onCommand resource:///modules/SmtpClient.jsm:513
    _parse resource:///modules/SmtpClient.jsm:370
    _onData resource:///modules/SmtpClient.jsm:418
    _onOpen resource:///modules/SmtpClient.jsm:397
    connect resource:///modules/SmtpClient.jsm:152
    sendMailMessage resource:///modules/SmtpService.jsm:92
    _deliverFileAsMail resource:///modules/MessageSend.jsm:1039
    _deliverMessage resource:///modules/MessageSend.jsm:757
    createAndSendMessage resource:///modules/MessageSend.jsm:138
    CompleteGenericSendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:4874
    GenericSendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:4810
    SendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:5329
    doCommand chrome://messenger/content/messengercompose/MsgComposeCommands.js:948
    doCommand chrome://messenger/content/messengercompose/MsgComposeCommands.js:1126
    goDoCommand chrome://global/content/globalOverlay.js:123
    oncommand chrome://messenger/content/messengercompose/messengercompose.xhtml:1
Invalid Console.maxLogLevelPref value: all
mailnews.send: Sending failed; Het bericht kon niet worden verzonden via uitgaande (SMTP-)server mail.zeelandnet.nl om een onbekende reden. Controleer of de instellingen van uw uitgaande (SMTP-)server juist zijn en probeer het opnieuw., exitCode=2153066725, originalMsgURI= MessageSend.jsm:321:27
    fail resource:///modules/MessageSend.jsm:321
    _deliveryExitProcessing resource:///modules/MessageSend.jsm:611
    sendDeliveryCallback resource:///modules/MessageSend.jsm:676
    OnStopRunningUrl resource:///modules/MessageSend.jsm:1293
    ondone resource:///modules/SmtpService.jsm:176
    _actionStream resource:///modules/SmtpClient.jsm:1306
    _onCommand resource:///modules/SmtpClient.jsm:513
    _parse resource:///modules/SmtpClient.jsm:370
    _onData resource:///modules/SmtpClient.jsm:418
    _onOpen resource:///modules/SmtpClient.jsm:397
    connect resource:///modules/SmtpClient.jsm:152
    sendMailMessage resource:///modules/SmtpService.jsm:92
    _deliverFileAsMail resource:///modules/MessageSend.jsm:1039
    _deliverMessage resource:///modules/MessageSend.jsm:757
    createAndSendMessage resource:///modules/MessageSend.jsm:138
    CompleteGenericSendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:4874
    GenericSendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:4810
    SendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:5329
    doCommand chrome://messenger/content/messengercompose/MsgComposeCommands.js:948
    doCommand chrome://messenger/content/messengercompose/MsgComposeCommands.js:1126
    goDoCommand chrome://global/content/globalOverlay.js:123
    oncommand chrome://messenger/content/messengercompose/messengercompose.xhtml:1
Prompter: internal dialogs not available in this context. Falling back to window prompt. Prompter.jsm:1084
    set modalType resource://gre/modules/Prompter.jsm:1084
    ModalPrompter resource://gre/modules/Prompter.jsm:1040
    getPrompt resource://gre/modules/Prompter.jsm:65
    getDefaultPrompt resource:///modules/MessageSend.jsm:300
    fail resource:///modules/MessageSend.jsm:319
    _deliveryExitProcessing resource:///modules/MessageSend.jsm:611
    sendDeliveryCallback resource:///modules/MessageSend.jsm:676
    OnStopRunningUrl resource:///modules/MessageSend.jsm:1293
    ondone resource:///modules/SmtpService.jsm:176
    _actionStream resource:///modules/SmtpClient.jsm:1306
    _onCommand resource:///modules/SmtpClient.jsm:513
    _parse resource:///modules/SmtpClient.jsm:370
    _onData resource:///modules/SmtpClient.jsm:418
    _onData self-hosted:1175
    (Async: EventHandlerNonNull)
    _onOpen resource:///modules/SmtpClient.jsm:397
    _onOpen self-hosted:1175
    (Async: EventHandlerNonNull)
    connect resource:///modules/SmtpClient.jsm:152
    sendMailMessage resource:///modules/SmtpService.jsm:92
    _deliverFileAsMail resource:///modules/MessageSend.jsm:1039
    _deliverMessage resource:///modules/MessageSend.jsm:757
    AsyncFunctionNext self-hosted:692
    (Async: async)
    createAndSendMessage resource:///modules/MessageSend.jsm:138
    AsyncFunctionNext self-hosted:692
    (Async: async)
    CompleteGenericSendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:4874
    GenericSendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:4810
    SendMessage chrome://messenger/content/messengercompose/MsgComposeCommands.js:5329
    doCommand chrome://messenger/content/messengercompose/MsgComposeCommands.js:948
    doCommand chrome://messenger/content/messengercompose/MsgComposeCommands.js:1126
    goDoCommand chrome://global/content/globalOverlay.js:123
    oncommand chrome://messenger/content/messengercompose/messengercompose.xhtml:1

I hope there is something in these logs that can help solve the issue.

I just disabled encryption and changed port 465 to 25. After this change I am able to send mail from Thunderbird 91 to a ZeelandNet server. This isn't a solution, because I don't want to advise to send mail without encryption. But maybe this helps in finding out where it goes wrong.

On the same machine Thunderbird 78.9.1 and Windows 10 Mail app are able to send mail to the same SMTP server (mail.zeelandnet.nl) on port 465 with SSL encryption enabled. It is just Thunderbird that needs to be turned back to no encryption and port 25.

Today I also got similar problem: not able to sent/smtp with image (embedded or attached). Same error message as JasperPouwelse.
I can:

• sent without image.
• send with image to my own mail-account.
• send from my mail-account if I use webmail and Iphones mail program. (Conclusion: the problem is with Thunderbird.)

I use:

• Windows 10
• Thunderbird 91.0.1 (updated from 91.0.0 today when problems occurred but did not solv it).
• smtp setting: port 465, SSL/TLS
• Kaspersky Internet Security

Attachment to comment no 10 by Jacob Schmidt in Bug 1726106.

Hi Jacob, your error message is different, notice the mailnews.smtp: Command failed: 550 This message was classified as SPAM and may not be delivered; currentAction=_actionStream SmtpClient.jsm:508:19 line. That means your SMTP server thinks the message is a spam. There are some other reports like this, see bug 1722916.

Hi Jasper, you should set the loglevel to All not all, and clear any existing logs before sending. The error is mailnews.smtp: Command failed: 501 <username-removed@zeelandnet.nl> BODY=8BITMIME SMTPUTF8: malformed address: BODY=8BITMIME SMTPUTF8 may not follow <username-removed@zeelandnet.nl>, I guess TB sends something like MAIL FROM:<username-removed@zeelandnet.nl> SIZE=123 BODY=8BITMIME SMTPUTF8 to your server, but get rejected. I don't see how this is related to encryption/port. Can you compare the MAIL FROM line of the two cases? Do you have any special chars (non-ascii) in your address?

(In reply to Ping Chen (:rnons) from comment #12)

Hi Jacob, your error message is different, notice the mailnews.smtp: Command failed: 550 This message was classified as SPAM and may not be delivered; currentAction=_actionStream SmtpClient.jsm:508:19 line. That means your SMTP server thinks the message is a spam. There are some other reports like this, see bug 1722916.

Hi Jasper, you should set the loglevel to All not all, and clear any existing logs before sending. The error is mailnews.smtp: Command failed: 501 <username-removed@zeelandnet.nl> BODY=8BITMIME SMTPUTF8: malformed address: BODY=8BITMIME SMTPUTF8 may not follow <username-removed@zeelandnet.nl>, I guess TB sends something like MAIL FROM:<username-removed@zeelandnet.nl> SIZE=123 BODY=8BITMIME SMTPUTF8 to your server, but get rejected. I don't see how this is related to encryption/port. Can you compare the MAIL FROM line of the two cases? Do you have any special chars (non-ascii) in your address?

Hi Ping, I just created new logs, with the loglevel at All instead of all. This is for port 25 without SSL:

Prompter: internal dialogs not available in this context. Falling back to window prompt. Prompter.jsm:1084
mailnews.smtp: Sending message <5918ff9e-2f04-99c5-e6a5-c7fe988c7302@zeelandnet.nl> SmtpService.jsm:84:18
mailnews.smtp: S: 220 zeelandnet.nl ESMTP ZeelandNet mailservice ready SmtpClient.jsm:417:17
mailnews.smtp: C: EHLO [192.168.1.23] SmtpClient.jsm:588:19
mailnews.smtp: S: 250-mail.zeelandnet.nl 250-SIZE 20480000 250-VRFY 250-ETRN 250-ENHANCEDSTATUSCODES 250-8BITMIME 250-DSN 250-STARTTLS 250 SMTPUTF8 SmtpClient.jsm:417:17
mailnews.smtp: Possible auth methods: SmtpClient.jsm:936:17
mailnews.smtp: Maximum allowed message size: 20480000 SmtpClient.jsm:942:19
mailnews.smtp: C: MAIL FROM:<username-removed@zeelandnet.nl> BODY=8BITMIME SMTPUTF8 SIZE=434 RET=FULL ENVID=<5918ff9e-2f04-99c5-e6a5-c7fe988c7302@zeelandnet.nl> SmtpClient.jsm:588:19
mailnews.smtp: S: 250 2.1.0 Ok SmtpClient.jsm:417:17
mailnews.smtp: MAIL FROM successful, proceeding with 1 recipients SmtpClient.jsm:1189:17
mailnews.smtp: Adding recipient... SmtpClient.jsm:1194:17
mailnews.smtp: C: RCPT TO:<jasper@profcom-it.nl> SmtpClient.jsm:588:19
mailnews.smtp: S: 250 2.1.5 Ok SmtpClient.jsm:417:17
mailnews.smtp: RCPT TO done, proceeding with payload SmtpClient.jsm:1243:19
mailnews.smtp: C: DATA SmtpClient.jsm:588:19
mailnews.smtp: S: 354 End data with <CR><LF>.<CR><LF> SmtpClient.jsm:417:17
mailnews.smtp: Sending 434 bytes of payload SmtpClient.jsm:564:17
mailnews.smtp: S: 250 2.0.0 Ok: queued as 4GqQhv3j4Zz38Vp SmtpClient.jsm:417:17
mailnews.smtp: Message sent successfully. SmtpClient.jsm:1302:21
mailnews.smtp: Closing connection... SmtpClient.jsm:171:17
mailnews.smtp: Idling while waiting for new connections... SmtpClient.jsm:1312:19
mailnews.smtp: Socket closed. SmtpClient.jsm:491:17

And this is for port 465 with SSL:
Prompter: internal dialogs not available in this context. Falling back to window prompt. Prompter.jsm:1084
mailnews.smtp: Sending message <ec687439-6a08-6f2e-4139-20b8764e2577@zeelandnet.nl> SmtpService.jsm:84:18
mailnews.smtp: S: 220 mail.zeelandnet.nl ESMTP Postfix SmtpClient.jsm:417:17
mailnews.smtp: C: EHLO [192.168.1.23] SmtpClient.jsm:588:19
mailnews.smtp: S: 250-mail.zeelandnet.nl 250-PIPELINING 250-SIZE 20480000 250-VRFY 250-ETRN 250-AUTH PLAIN LOGIN 250-AUTH=PLAIN LOGIN 250-ENHANCEDSTATUSCODES 250-8BITMIME 250-DSN 250 SMTPUTF8 SmtpClient.jsm:417:17
mailnews.smtp: Possible auth methods: PLAIN,LOGIN SmtpClient.jsm:936:17
mailnews.smtp: Maximum allowed message size: 20480000 SmtpClient.jsm:942:19
mailnews.smtp: Current auth method: PLAIN SmtpClient.jsm:654:17
mailnews.smtp: Authentication via AUTH PLAIN SmtpClient.jsm:669:21
mailnews.smtp: C: Logging suppressed (it probably contained auth information) SmtpClient.jsm:584:19
mailnews.smtp: S: 235 2.7.0 Authentication successful SmtpClient.jsm:417:17
mailnews.smtp: Authentication successful. SmtpClient.jsm:1146:17
mailnews.smtp: C: MAIL FROM:<username-removed@zeelandnet.nl> BODY=8BITMIME SMTPUTF8 SIZE=434 RET=FULL ENVID=<ec687439-6a08-6f2e-4139-20b8764e2577@zeelandnet.nl> SmtpClient.jsm:588:19
mailnews.smtp: S: 250 2.1.0 Ok SmtpClient.jsm:417:17
mailnews.smtp: MAIL FROM successful, proceeding with 1 recipients SmtpClient.jsm:1189:17
mailnews.smtp: Adding recipient... SmtpClient.jsm:1194:17
mailnews.smtp: C: RCPT TO:<jasper@profcom-it.nl> SmtpClient.jsm:588:19
mailnews.smtp: S: 250 2.1.5 Ok SmtpClient.jsm:417:17
mailnews.smtp: RCPT TO done, proceeding with payload SmtpClient.jsm:1243:19
mailnews.smtp: C: DATA SmtpClient.jsm:588:19
mailnews.smtp: S: 354 End data with <CR><LF>.<CR><LF> SmtpClient.jsm:417:17
mailnews.smtp: Sending 434 bytes of payload SmtpClient.jsm:564:17
mailnews.smtp: S: 501 <username-removed@zeelandnet.nl> BODY=8BITMIME SMTPUTF8: malformed address: BODY=8BITMIME SMTPUTF8 may not follow <username-removed@zeelandnet.nl> SmtpClient.jsm:417:17
mailnews.smtp: Command failed: 501 <username-removed@zeelandnet.nl> BODY=8BITMIME SMTPUTF8: malformed address: BODY=8BITMIME SMTPUTF8 may not follow <username-removed@zeelandnet.nl> ; currentAction=_actionStream SmtpClient.jsm:508:19
mailnews.smtp: Message sending failed. SmtpClient.jsm:1300:21
mailnews.send: Sending failed; Het bericht kon niet worden verzonden via uitgaande (SMTP-)server mail.zeelandnet.nl om een onbekende reden. Controleer of de instellingen van uw uitgaande (SMTP-)server juist zijn en probeer het opnieuw., exitCode=2153066725, originalMsgURI= MessageSend.jsm:321:27
Prompter: internal dialogs not available in this context. Falling back to window prompt. Prompter.jsm:1084

The only difference I can see in the MAIL FORM line is the FULL ENVID. The senders address does not contain any special characters. Also, I have by now seen this at about 10 customers, all with the combination Thunderbird / ZeelandNet. These customers all use their own personal address to send mail, so these are all different addresses.

Are the failed email addresses using extended characters? The error is the address is invalid, you have removed the actual addresses used, so the obvious question is what characters would be in a typical failing address.

I don't understand, in the case of 465, your server accepted the MAIL FROM first, but after a while returned an error

C: MAIL FROM:<username-removed@zeelandnet.nl> BODY=8BITMIME SMTPUTF8 SIZE=434 RET=FULL ENVID=<ec687439-6a08-6f2e-4139-20b8764e2577@zeelandnet.nl> 
S: 250 2.1.0 Ok 
// after a few more exchanges...
S: 501 <username-removed@zeelandnet.nl> BODY=8BITMIME SMTPUTF8: malformed address: BODY=8BITMIME SMTPUTF8 may not follow <username-removed@zeelandnet.nl> 

Can you get some SMTP logs from TB78 as a comparison. Start TB in cmd with MOZ_LOG=SMTP:5 ./thunderbird.exe

(In reply to Matt from comment #15)

Are the failed email addresses using extended characters? The error is the address is invalid, you have removed the actual addresses used, so the obvious question is what characters would be in a typical failing address.

Hi Matt, the part I removed (before the @) contains only letters. I also checked the addresses of the first 5 customers who reported this issue, they all have only letters before the @zeelandnet.nl

Hi Ping, I just installed 78.9.1 and created the log above. This is with 78.9.1 with SSL enabled and port 465

Thanks, that's very helpful. Can you please do another test with 91? Set mailnews.smtp.jsmodule to false, restart TB with MOZ_LOG=SMTP:5 ./thunderbird.exe, get the logs by sending a mail.

This section from RFC6531 seems very relevant to this bug

If the SMTPUTF8-aware SMTP
client is aware that neither the envelope nor the message being sent
requires any of the SMTPUTF8 extension capabilities, it SHOULD NOT
supply the SMTPUTF8 parameter with the MAIL command.

Because there is no guarantee that a next-hop SMTP server will
support the SMTPUTF8 extension, use of the SMTPUTF8 extension always
carries a risk of transmission failure.

It looks like you have more than one SMTP servers, when using port 465, your first server accepted the MAIL FROM ... SMTPUTF8 ..., but your second server rejected it a bit later. Can you turn off the SMTPUTF8 capability on your first server?

Thanks, this is related to changes in bug 1563891. I will make a fix soon, but it takes time to ship it to 91.

Hi Ping, Thanks a lot! I understand this takes some time. Just so I can keep my customers informed, can you give a rough estimate how long this will take?

In your previous post you wrote: 'Can you turn off the SMTPUTF8 capability on your first server?' Unfortunately I cannot make changes on the server. I am not a ZeelandNet employee. Our company helps customers with solving their IT issues. Because we are located in the Dutch province of Zeeland a lot of our customers are also customers of ZeelandNet, the regional ISP. Is it wrong that they have this capability enabled on their server? If that is the case I can try to contact them about this.

Also fix unneeded DSN part in the MAIL command.

can you give a rough estimate how long this will take?

Next month or maybe October, updates will be posted here, you will receive notifications.

Is it wrong that they have this capability enabled on their server?

Looks wrong to me, since it accepted the MAIL FROM at first. Either disable it or enable it on all their servers. By removing the capability from the server, TB won't send the SMTPUTF8 as well.

Pushed by mkmelin@iki.fi:
https://hg.mozilla.org/comm-central/rev/5ac04ed3a912
Do not send unneeded SMTPUTF8 in MAIL command in SmtpClient.jsm. r=mkmelin

Not sending SMTPUTF8 when not required was discussed at length in bug 1710224 comment #71 and below. Sadly it wasn't addressed back then. For TB 91 you potentially need a special patch to also support the old C++ module in case users switch to it.

That would assume people run into multiple bugs for the same server, so let's not go there unless we have a pressing need to do so later.

(In reply to Ping Chen (:rnons) from comment #26)

can you give a rough estimate how long this will take?

Next month or maybe October, updates will be posted here, you will receive notifications.

Assuming no complications arise, with Ping's guidance on the patch, I would anticipate in beta next week, and 91.1.0 in early September.

Comment on attachment 9236976 [details]
Bug 1726106 - Do not send unneeded SMTPUTF8 in MAIL command in SmtpClient.jsm. r=mkmelin

[Approval Request Comment]
Regression caused by (bug #): bug 1563891
User impact if declined: Sending fails for some servers with SMTPUTF8 capability but not properly configured
Testing completed (on c-c, etc.): c-c
Risk to taking this patch (and alternatives if risky): low, most servers don't mind it

I just received a message from ZeelandNet in response to my message about this issue. They have changed the settings on their servers. My translation:

"The problem is the spamfilter behind the mailserver does not support SMTPUTF8 but the mailserver itself does. We changed this so SMTPUTF8 support is no longer announced on port 465 and 587. The issue should now be resolved."

On my test-pc I am now able to send mail from Thunderbird 91 over the ZeelandNet servers and I have received response from 2 customers that this solved the issue for them.

Comment on attachment 9236976 [details]
Bug 1726106 - Do not send unneeded SMTPUTF8 in MAIL command in SmtpClient.jsm. r=mkmelin

[Triage Comment]
Approved for beta

Thunderbird 92.0b3:
https://hg.mozilla.org/releases/comm-beta/rev/021d6807b033

Comment on attachment 9236976 [details]
Bug 1726106 - Do not send unneeded SMTPUTF8 in MAIL command in SmtpClient.jsm. r=mkmelin

[Triage Comment]
Approved for esr91

Thunderbird 91.0.3:
https://hg.mozilla.org/releases/comm-esr91/rev/4506735acc57