This is a commercially signed certificate.
T-Bird throws the error on an attempted send. I have no problems otherwise, and have used several email clients to send and receive email on this server using pop3s on 995 and smtps on 465.
When I asked what ports to use in these forums earlier I was completely ignored, so I assumed nobody ever used SSL ports with Zimbra, which isn't acceptable in our environment (server shared between internal and outside users).
Zimbra is configured to use 465, I wish I could remember where that was confirmed for me, on the console or in the admin console, but it did confirm it and I have been able to send email with Outlook (both with and without the Outlook connector), OE and The Bat. Of course ZDC and the webmail work.
There is more detail posted here:
T-Bird problem, or CA, or *gasp* maybe something I did? • mozillaZine Forums
Cheers,