Thank you! You were correct!

It was due to the fact that I set Thunderbird to accept permanently the previous certificate generated by my prevoius zimbra installation.

Problem solved!
Thanks...

> did you let zimbra installer generate it's own certificate, or did you install one afterwards?

I let zimbra's installer generate its own certificate - but I still get this same error after...

I've installed zimbra, and its been up and running fine.
I can connect through IMAP with Thunderbird using TLS, and the webmail works wonderfully.

The current problem is that I can't send emails...