I gave a user in exchange 2007 (latest en updates en service packs) a extra e-mail address. So currently the user has firstname.lastname@example.org and email@example.com. Both of them work to receive e-mail. However he seemed to can only send from domaina.com but not domainb.com. He always gets the error he cannot send in behalf of. He only has the domainb.com e-mail address, no one else is using it.