[Dovecot] Thunderbird STARTTLS error

[Markus Fritz]

Hello,

the error is still present:
May  8 19:47:18 opsys dovecot: imap-login: Disconnected (no auth 
attempts): rip=82.113.119.140, lip=78.46.216.126

Whenever I start a session with openssl to STARTTTL (Server: 
mail.opsys.de) the handshake is successfull. Also I am able to login to 
my account via 1 login.
In Thunderbird port 993 for SSL/TLS works correct, only STARTTLS on port 
143 isn't working properly.
The cert is Class 1 and signed by StartCom Ltd..
Dovecot.conf (for viewable reasons of this mail pasted): 
http://pastie.org/private/bmrymyuo16ohzxdahf0nq
And here openssl output: http://pastie.org/private/3rpgll2s7hblev9ozpcq8w
Note the 'Verify return code: 21 (unable to verify the first 
certificate)' in the output...

Thanks for helping, I am working on this problem since 3 days.

Kind regards

Markus Fritz