Thunderbird certificate error

[G_]

Since a couple of weeks I get a 'send message error' whenever I try to send mail. (My primary pc doesnt have avast installed and doesnt have this problem, hence the late post; this also means that I dont now what caused it - Windows, TB, Avast update?)

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Send message error

Sending of the message failed.
Unable to communicate securely with peer: requested domain name does not match the server’s certificate.
The configuration related to mail.<domain>.com must be corrected.
I can confirm the security exception and send emails, but the error returns after a reboot

I've tried:
- https://support.avast.com/en-ww/article/Troubleshoot-invalid-email-certificate/
Exporting the mail shield certificate and importing in TBird. Result: error saying it's already installed

- removing and re-adding the certificates

- https://forum.avast.com/index.php?topic=238173.msg1561545#msg1561545
Checking the option 'This certificate can identify websites', but it was already checked

- https://forum.avast.com/index.php?topic=224365.msg1567777#msg1567777
Installing the certificate in Local computer certificate manager

- Deleting the certificate(s) in TB under 'Certificate Manager - servers'. (A new one was "generated by Avast Antivirus for SSL/TLS scanning" and installted the next time I sent an email.)

Is there anything left I can try, or should I just switch to a different AV?

Unrelated: who thought this PoS verification method was a good idea? I probbly mistyped, because it is unreadable, but I got no error, nothing was posted and all text was deleted. (Luckily, I developed a habit of copying my text before I submit.)

[Asyn]

- Which Avast..? (Free/Premium)
- Which version/build of Avast..?
- OS..? (32/64 Bit..? - which SP/Build..?)
- Other security related software installed..?
- Which AV(s) did you use before Avast..?
- Which version/build of Thunderbird..?

PS: Captcha is only needed for your first 3 posts. (Spam protection)

[G_]

- Which Avast..? (Free/Premium)
- Which version/build of Avast..?
- OS..? (32/64 Bit..? - which SP/Build..?)
- Other security related software installed..?
- Which AV(s) did you use before Avast..?
- Which version/build of Thunderbird..?

PS: Captcha is only needed for your first 3 posts. (Spam protection)

Thanks for the quick reply!

Avast Free, Win 10 x64, TBird x32 (all 3 up-to-date/latest versions)
No other security-related software.

Note: Both computers are configured the same, the only difference is Avast on PC2

[Asyn]

Does it work if you disable mail shield..?

[G_]

I'm now one step closer; apparently something changed at the host and I had to change my server settings from mail.<domain>.com to mail.<host>.com.

(Note: PC1 doesnt have Avast installed, no TBird problems on that one. I'll call folders from PC1 e.g. Inbox1; folders from PC2 e.g. Drafts2)

After confirming the securrity exception I can now send emails (to myself), and they arrive in Inbox1, but not Inbox2, nor can they be saved in Sent2:

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Your message was sent but a copy was not placed in your sent folder (Sent) due to network or file access errors.
You can retry or save the message locally to Local Folders/Sent-mail@<domain>.com.
If I then disable the mail shield, everything works normally.

However, if I enable the shield again, it doesnt work like before:
I can send emails, they show up in Inbox2 and 1, Sent items will now be saved on the server (they show up in Sent1), but not locally. So: Inbox1 and Drafts1 sync, but not Sent1, Archives1 or Trash1.

I've disabled the shield indefinitely, rebooted, and succesfully sent an email. But if I turn the shield on again, it again asks me to add the security exception and I'm back to a non-syncing Sent2.

Edit: "The letters you typed don't match the letters that were shown in the picture. " x1 x2 x3 x4

[Asyn]

1. Download Avast Free Antivirus: https://files.avast.com/iavs9x/avast_free_antivirus_setup_offline.exe
2. Follow instructions: https://www.avast.com/uninstall-utility (Run this tool for all prior installed Avast versions..!!)
3. Reinstall Avast with the downloaded installer from point 1.
4. Reboot.

[G_]

Reinstalling did the trick!

Not immediately: after reinstall and reboot it still didnt work, so I disabled the shield, test&succeed, enable, test and fail, reboot, test&fail, disable, test&succeed, enable, test&succeed! (And reboot, test&succeed again.) This was quite the frustrating journey, but it seems to be solved now! Thanks for the help, Asyn!

[Asyn]

You're welcome.