Finding mail problems

Dear all,

I would appreciate any help in finding the best places to look when
there are complaints about people missing messages on lists that I
manage.

I've turned on verbose logging on one of the lists, but there's no
sign of it being unable to send messages to any of the people.  I've
sent a test message and asked people to reply to me if they receive
it, so that I can get some idea of who isn't getting messages.

The problem addresses seem to vary, judging by what people are saying
- they receive some messages but not others.

Any clues gratefully received.

Many thanks,

'ö-Dzin Tridral