When setting up secondary mailboxes in Exchange 2003, and giving users permissions to send from that account ( I can't remember the exact steps I took), if they specify the email in 'From:' field, the email won't send. The user gets the automated no permission to send to recipient. If they chose the account via the address book, which has that email address, it works. This is a pattern, not just a specific account.
Anyone know what might be happening here?