I created a POP account in Outlook. (2003, 2007 and 2010)
I checked the MX record, and the server settings.
I can send email using OWA (Internet explorer mail client).
thus I know the server is working.
I believe the error is with Outlook settings: System Administrator reposne:
The following recipient(s) cannot be reached:
'firstname.lastname@example.org' on 7/2/2012 5:35 PM
Server error: '550 No such domain at this location'