Why this strange construction for aditional administrators? dont get me wrong it is a workarround only you also need to open another email and you need to maintain that email account.
A mail group it self is just a reception email, it receives emails and forwards them to the predefined emails underneath it ..There isn't much to maintain with it.
The forward mail server is not a plesk mail server it is our coperate exchange server, it also means i need to create mailboxes for every server we have to send to the right people.