The ca directory contained extra links to different certificates. This seemed to not bother 5.0.9 but under 5.0.10 postfix has a fit if there are more then 3 files in that directory. Check out
Error (MTA): Unable to set STARTTLS - Zimbra :: Wiki.
As per support instructions the fix was:
1) Clear all the contents of the /opt/zimbra/conf/ca directory by backing them up
somewhere on disk.
2) Copy the /opt/zimbra/ssl/zimbra/commercial/commercial.key /opt/zimbra/conf/ca/ca.key
3) Copy /opt/zimbra/ssl/zimbra/commercial/commercial.crt /opt/zimbra/conf/ca/ca.pem
4) Create the hash value
ln -f -s ca.pem /opt/zimbra/conf/ca/`openssl x509 -hash -noout -in
/opt/zimbra/conf/ca/ca.pem`.0
5) Chmod 644 /opt/zimbra/conf/ca/*
6) Restart the zmcontrol