I just discovered that, because of some changes I made to my accounts a few days ago, a lot of my incoming email has been bouncing over the weekend.
Due to an undocumented ‘feature’ of my domain host, I had somehow managed to set a couple of email addresses to forward in a perpetual loop to themselves; never quite making it as far as the mailbox.
Oh well, the error has now been corrected, and hopefully I didn’t miss anything too important.