One of my clients runs SBS2008 with Exchange 2007... One of the employees has multiple mailboxes, all but one of these domains the server is authoritative for, however one isn't and that particular domain address he receives emails forwarded from another server to him (external and not part of the company network in any way), he can also send using that address, out through their exchange server no problem BUT if he tries to copy himself with that address it immediately bounces with the following error:
Delivery has failed to these recipients or distribution lists:
The recipient's e-mail address was not found in the recipient's e-mail system. Microsoft Exchange will not try to redeliver this message for you. Please check the e-mail address and try resending this message, or provide the following diagnostic text to your system administrator
#550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##
To complicate things slightly, this address also happens to be his default email address for his exchange account as well, i.e. when he sends an email outlook will always use this address as default (unless of course he chooses a different one manually)...