This is the present setup of our system:
Outgoing Mail: All Mail is sent via our ISPs smtp server.
Incoming Mail: We receive mail on SendMail for our domain and then relay this to our local Mail Server.

We wish to copy all incoming and outgoing messages from and for certain accounts to gmail accounts we have setup.
However we do not wish the copies to be sent via our ISPs mail server.
How can we achieve this?