Postfix

Relay all mail to an external account which can come in handy in your lab environment. ...

I hit an intriguing problem as I needed to relay selected mailboxes rather than the whole domain. I assume that you have already noticed that everything needs to happen on the same domain. It is rare and quite interesting task so I will quickly describe it here using ISPConfig 3 but beware as this is a very specific topic so I strongly suggest to investigate it further before applying any changes. ...