Exchange went weird. One of the two storage servers decided it couldn't use DNS for some weird reason. I could resolve everything from command-line just peachy, and ipconfig /flushdns didn't do its usual magic. The smtp queue (didn't know it used that as a transport on this level) to the OTHER Exchange store had 1600 messages stacked up in the two hours it had been unresponsive. Unfortunately, a reboot was needed to kick it into gear. Which we gave it, and mail started flowing again.

Silly thing.