I have an Exchange 2007 organization that contains multiple SMTP domains. AD is configured so that each user has multiple e-mail addresses, one from each SMTP domain. Most of the time, all is well. However, sometimes, (more often than I'd like) if an internal user tries to send a message to another internal user using the alternate address, we get an NDR that says the user does not exist. Even more interesting is the hash it generates in the NDR of the recipient it is trying to find. It tries to query a user like so: user.namesecond_domain@first_domain.local. Of course the lookup will fail this way, but where does it get that address, and why does it work as expected most of the time?