At 3AM on September 1, we began having issues with the hardware hosting our mail. We are currently working on identifying the problem and minimizing its impact upon the rest of our services.
We will update this post once we have more information.
UPDATE: It looks like our disk array had some hard drive issues. We were using mirrored hard drives, so we don't think any data was affected. Everything should be working again, but please let us know if you see anything out of the ordinary.