There was an SMTP communication problem with the recipient's email server. Please contact your system administrator.
Now I was getting these from emailing my father. His ISP has *the* most agressive spam filtering I've ever seen. Naturally, like every other time, I was quick to jump on the bandwagon of blaming my father's ISP. Heck, he'd already lost 3-4 known real emails to this overly agressive spam filter. So I email them to complain.
At work that day, I decided it was time to check my home email (OWA comes in handy there) and I couldn't resolve my domain to save my life! Fortunately I have two domains pointed at my SBS box, so I used that one as a backup and was able to get to OWA. I started seeing the same message when emailing some of my friends.
Something was in fact wrong on my side.
Logging into my dynamic DNS server, I saw nothing astray, so I decided that I should email their support to ask for help. Then I had a thought. When the heck does this domain expire? Don't ask me why I thought of that. But sure enough *yesterday* was the day my domain expired.
So here is the bottom line. Computer's remember things. Right up to the minute if you ask them. Human's don't. Put this kind of stuff in your calendar so you get a reminder and your site isn't down for 15 hours while DNS replicates across the Internet.