I have an Exchange 2007 server that is authoritative for two domains. One user has an email address for both domains (firstname.lastname@example.org and email@example.com are both aliased to his user). His default address is firstname.lastname@example.org which is set up using an exchange account set up in Outlook 2007. This user would like to be able to send out of either email address by choice. I created a dummy pop3 account in his Outlook 2007, but I've run into an issue.
If I set outgoing authorization in the dummy POP account to nothing, he can send a test email to himself through the dummy account just fine. However, if I try to send an email to anyone else, I get an NDR stating "Unable to relay." If I try to put in his user account details into the dummy POP account's outgoing authorization, I just get a popup asking for a username and password over and over again, though I am certain the username and password is correct. I've tried multiple entries for the username including domain\username, email@example.com, and username. Does anyone have any ideas on why this is happening?