Tony, I think you're missing something here.
ISP's pretty much always have
secondary and tertiary mail servers. Just taking down ONE server doesn't
make your mail bounce. The classiccmp server would try the secondary. For
example, your ISP (demon.co.uk) has two mail servers set up. Surely they
don't take BOTH of them down for maintenance at the same time ;)
There ahve been a few times when demon has (AFAIK) accepted no incoming
mail at all. Maybe because both servers are down, maybe because of
network problems (which may not actually be the fault of demon),
maybe.... I don't try to understand what the problem is, I have enough
trouble keeping my machines running, without bothering about machines I
can't get to fix anyway...
-tony