I'm feeling a bit dense today so any help welcome!
Essentially, I've just noticed that local mail hasn't been delivered for
a couple of weeks. I can email off my box but not to my username on the
box. I can't see what the problem is. They are probably both a red
herring  but (a) I did have some DNS problems just prior to the last
received email and (b) switched off the box and physically moved it to a
new location (and the new IP number) just after the last received email.
I'm unsure how to go about debugging this so all pointers welcome!