Sockets?..
Simple, create a server.. and then the clients.. Server will receive the mail, see the destination, and send it to it.. Not really a hard task..
Don't except a full code on it... Google on socket/tcp-ip tutorials and so..
That or use somekind of Database system.. but it might be slower.. dunno.. i wouldn't use a DB as Client-Server on this one..
Or create a server and a java script webpage (example).
At the server where you can access with and send e-mails (with support of a server coded in VB or so...).