I set up my mail server with Postfix (MTA) and Dovecot (IMAP, POP3).
When I tested my installation with an open source webmail interface everthing has been working well:
login with username and password, receive mails (port 993, ssl/tls) and send mails (port 587, starttls).
Today I started testing local clients beginning with Thunderbird.
Here I get an error message like this: Click Me.

In Dovecot I have already changed

ssl_protocols = !SSLv3 !SSLv2
to
ssl_protocols = TLSv1.2 TLSv1.1 TLSv1 !SSLv3 !SSLv2

Moreover I tried to deactivate the OCSP certificate validation in Thunderbird
(Settings > Enhanced > Certificates > Validation)

But the issue keeps existing :(
Anyone here to help me please?