MSXch – custom Recipient won’t send????

When I create a custom Recipient (Internet Email) in the GAL, and then try to send mail to that recipient – someonespecific@somewherespecific.com – Xchange 5.5 SP3 immediately sends an ndr back saying ‘Recipient can’t be found’
This happens with only 2 Inet email addresses. could it be when I was playing ‘HELO’s’ when battling with Relaying??? – this is getting absurd. Other Inet email addresses will send.
Any help greatly appreciated…..