Interesting this one. I get notifications (PMs and new members joining) so clearly the forum is configured to use the mailserver OK. However, maybe the mailserver itself isn't playing ball.
Julian I've sent you a message manually via the mailserver - it won't have a subject (couldn't remember the commands for that!) but should otherwise be OK.
There aren't any warnings/errors in the mailserver log for the last few days, so this is bit of a mystery.