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Dear Denis,

Thank you for posting your question on our forum.

This issue is TLS certificate related, you can fix it by following steps below:

1. Make sure that the TLS protocol is enabled: W+S => Internet Options => Advanced.

2. If you are currently using the Thunderbird 78+ version, TLS 1.0 and TLS 1.1 are not supported by the application. Please select TLS 1.2 and higher.

3. Some antiviruses may use their self-signed certificates, so we recommend checking the current TLS certificate in case it’s expired: Thunderbird => Settings - Privacy & Security - Manage certificates.

Should you have any other questions or need further assistance, please do not hesitate to contact us.

Thank you so much. I will try to apply your recommendations, and come back to you if the problem persists.
Sincerely.

Since yesterday that I selected TLS 1.3 in addition to TLS 1.2, on your indications, the error message did not return.
I can't be sure, but it looks like the problem is solved.
Thanks again.

Hello Denis,

Good news!

If, by any chance, the problem occurs again, please do not hesitate to contact us.

Hello Vera,

I regret to come back to it, but despite the settings you advised me, the error window appears from time to time. This does not prevent the reception and sending of mails, I just report it for information, as a problem not yet solved.
Thank you anyway for your answers. Best regards.

Hello Denis,

Sorry about that.

Looks like one of the installed certificates has expired after all. Could you please specify if you checked the certificates in Thunderbird (point 3 in our earlier recommendation)?

Hello Vera,

Yes, I checked, the items are not quite in the same order, but the same are checked.

Hello Denis,

Have you found any information indicating expiration of the certificates? You can send us a screenshot if you’d like us to take a look.

Thank you.

Thanks Vera,
I don't know anything about certificate management, but the error window always indicates that the TLS version or the certificate used by the imap.sfr.fr server is incompatible.
imap.sfr.fr is the imap server of 3 of my 7 mail accounts installed on Thunderbird.
But here, to answer your indications, two screen views of my settings, in the internet options and in the Thunderbird settings:

Thank you anyway ! Best regards.

Hello Denis,

Could you please try clicking on "Gérer les certificates" to see if there is any informaition on available updates?

If you are unsure about certificate management, it might also be useful to consult with Thunderbird support, they should be able to guide you through the necessary steps.

Hello Vera,
It was a bit tedious, but I made screen views of the window obtained from "Manage Certificates" (4 empty tabs, and I unfolded the last tab).
Thank you in advance if this means anything to you.
Best regards.

Here is the continuation:

Here is the continuation:

Hello Vera,
I'm sorry for sending you so much information on a subject that wasn't blocking my emails.
I did some Thunderbird troubleshooting today and it seems like the mysterious issue is gone.
If he comes back, I will reopen this discussion.
Thanks a lot for your answers. Best regards.

Dear Denis,

Great news! Just in case the issue occurs again, here is some additional tips we've put together for you:

1. Make sure that your system time is correct (sometimes, even a slight discrepancy may cause the issue).

2. In the list of certificates, locate the one belonging to sfr.fr, press Voir and check the content for any information on the certificate’s expiration date.

3. Check your Account settings (Thunderbird => Settings) to see what values are selected under Security settings => Server settings. Try changing them and see if it helps.

4. Roll Thunderbird back, to a version earlier than 78th, and check if it helps.

Please let us know if there is anything else we can assist you with.

Hello Vera, thank you very much for this feedback.
In fact, the error window reappeared (a little less often), but I didn't have much time to take care of it.
I will try your instructions in a few days and will keep you informed.
Best regards.