Exchange 2010 has a distribution group setup called postmaster and the Email addresses associated with it are email@example.com, firstname.lastname@example.org etc
Internal emails sent to postmaster work fine and go to all members of the group.
However from external via SMTP I get an 550 5.1.1 error saying user unknown.
What have I missed?
All my other Dist Groups work internally and externally, its just this one.