I'm using an mta relay host for delivering email at the outside. This relay is configured without ssl or tls.

But lately I found that I could use ssl with smtp.gmail.com and done the configuration. But this way I've got the following unwanted behavior:
All outgoing messages got the 'return-path' and the 'from' fields with the authentification user used to connect to the gmail.com smtp relay host.
So the result is if you send a message from zimbra with say the account user@domain.com then the reciever will get the message with a return address like aguy@gmail.com.

Is this a special behvior done by gmail.com or would this append for all mta relay host using ssl?