If you’re using Eset security on your PC and Thunderbird for an email client they will have stopped working on 1 December.
The simple workaround was to disable Eset (webmail and email filtering).
There is a permanent solution and it’s pretty simple.
It was all to do with Eset replacing their security certificate. Thunderbird didn’t know, equalling a certificate mismatch and a block against the mail service.
The Eset forum was quick to fill with troubled users and equally, a simple solution that replaces the certificate. The method described in the weblinks below has been deployed on multiple client computers with 100% success.
First step involves removing the outdated certificate from Thunderbird, then closing it.
Second step involves disabling SSL/TLS protocol filtering in Eset, then re-enabling it.
Third step involves opening Thunderbird and checking the Eset certificate has updated.
Background and initial steps:
https://forum.eset.com/topic/26517-problems-with-thunderbird-and-imaps/page/2/
Eset – how to do the second step:
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