yes, they are very open.
> I'm pretty sure though you won't be able to use exim, it's pretty heavily integrated with postfix's configuration
I don't think so. Zimbra includes an own LMTP server (as far as I know).
Zimbra does include a postfix rpm with an example configuration, but you
can use every mailserver that talks to Zimbra by LMTP.
However, I don't think that you will be able to use Mysql, as Zimbra
manages all user data by LDAP.
