It turned out that this was a combination of two things. One I was calling
the machine by its full name in /etc/aliases which meant that even though it
decided it was local it changed its mind later. Second that the mx record
for it had been deleted from the DNS (by my ISP who provides this service).
It was deciding that it was local, passing it to the system alias director,
deciding it was external, trying to resolve the host, and getting very
unhappy and bouncing it as unroutable. When I managed to get the DNS up
again it passed it over to the local delivery for tidy up and it all worked.
I have now changed things a bit so it doesn't have to go and look outside to
see if it is still there. Many thanks to all who helped (I had forgotten the
debug option <fx: fly swatter hitting baron greenback>).
Cheers,
Rufus
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