I've set up quite a few aliases in SBS 2008 and all appear to work except one, which gets an unknown address NDR when emails are sent from one sender.
SMTP error from remote mail server after RCPT TO:<firstname.lastname@example.org>:
host remote.otherdomain.co.uk [81.xxx.xxx.xxx]:
550 5.1.1 User unknown
It works perfectly when I send one.