I had a customer some time ago who always spelled out my full company name when sending  me email. I finally registered mcbsystems.com so I could receive her emails.

Since then I've implemented Exchange 2003 (under SBS 2003), but apparently I never tested the domain alias until I was setting up Postini. It turns out that Exchange was bouncing messages sent to the domain with a "550 5.7.1 Unable to relay" message.

My situation  pretty much matches the second "class" of SMTP domain in this Microsoft Knowledge Base article:

Setting up SMTP domains for inbound and relay e-mail in Exchange 2000 Server and in Exchange Server 2003

Howver, whereas the article suggests changing the SMTP connector to allow messages to be relayed to this domain, I discovered another option. If I go to Recipient Policies > Default Policy > E-Mail Addresses (Policy) tab, create a policy @mcbsystems.com, and check the box to the left that causes it to auto-generate the address for new Active Directory users, relaying works. If you don't check the box for auto-generating addresses, relaying only works if it is set up in the SMTP connector. I'm not sure what auto-generation of addresses has to do with relaying, but I thought I'd better write this down before I forget it!