I think we are on the wrong page here. There is no option for a user to use the external server's smtp. so the presented address will always be login@yourserver.xxx. even if you change the reply to information it will not be the same as using the external smtp that you are supposed to use with an external account. I am not talking about relaying, I am talking about a direct connection. Such as. If I set up my gmail imap account then I would like to be able to use Zimbra as a full client and not just a "mail receiver." Maybe people do not want to use their Zimbra address to reply to gmail msgs.
Plus. I would expect people in my company to use such things with professionalism.

If anything. I believe this should be added on a per user basis. Where you can grant access to external smtp servers from the admin console in user acct settings.