For some reason, forwarding to external smtp addresses has stopped working for mail-enabled AD users that do not have an Exchange mailbox.
I used to be able to define a user without a mailbox, then create an e-mail address for the user. The external address would be the primary and I would define a secondary address for the user from our smtp domain. This way, the user could receive mail sent to his (fictitious) address on our smtp domain at his external address, without storing the messages on my Exchange server, or defining another user. Somehow, sometime, it stopped working.
We use Exchange 2003, SP2 and our Exchange server receives mails for 2 smtp domains.