glimbeek - 7:08 am on Feb 9, 2010 (gmt 0)
Not sure I'm getting you kaled, but I used your example:
firstname.lastname@example.org and email@example.com ofcourse with the email address that's not working not with example1.com :P
And the firstname.lastname@example.org fails with the "550 "Unknown User"" error and the email@example.com fails with a "Mail delivery failed: returning message to sender".
Not sure what this means though...