Secondary Exchange Mailbox From: problem

I've been searching some but Can't seem to figure this out. When someone adds a department mailbox to their outlook 2007 and send from it the recipient will see the email coming from that very person. I'm trying to figure out how to make any messages sent from that secondary mailbox to auto send as department@company.com instead of Jim@company.com without having to go to the From button and picking it every time they need to send a new message.