Zimbra is great, but it have serious and annoying problems...

For example: I have one Zimbra server for the domain "mycompany.com", but any user of my system can send a e-mail as user@gmail.com or billgates@msn.com through MY ZIMBRA MTA! This is so bad... SO BAD! It is terrible, any good mail server does not allow such things happen... never!

So, how do I restrict the "mail from" command to allow outgoing mails ONLY FROM the domains configured at MY ZIMBRA?!

I'm looking for smtpd_sender_restrictions option at the file zmmta.cf, I'm trying everything to block this behavior, without success...