I am a muppet.

Due to sleep deprivation and horrible misreading of email headers, last night I accidentally managed to report my domain as a spammer. Currently, neither my domain name nor any emails associated with it are working. Oops.

On the plus side, at least I'm not getting 800+ emails a day of rejected spam caused by spammers putting my domain in as the "reply to".

Hopefully the hosting company will fix things shortly.