Gmail as Relayhost

[Dominique]

Hi,

I am looking at using gmail as a relayhost in our current server setup 
ubuntu12.04/postfix/cyrus instead of using the ISP relayhost.

I have it working, but the outgoing email address is replaced by the 
gmail address (from the authentication info) - things that did not 
happen when using the ISP.

How can I fix that ?

Thanks,

Dominique