Apparently it isn't my configuration that is wrong (well it is, but that isn't what is causing the problem). It appears that Blueyonder are at it again. Some spammer is sending a "dictionary attack" and has overloaded the mail servers. It would be funny if it wasn't for the fact that this happened to Blueyonder last year and they don't seem to have learned any lessons from it. Meanwhile all the email I thought had been eaten is slowly filtering into my inbox including the test emails one of my fellow IRC-ers send on Saturday night.
Technology, don't you just love it.