Hi Matrix,
Yes the same test message sent many, many times to "good" and "bad" addresses and there seems to be no common factor on the failed ones.
I am convinced it is not my email server or why could I get them through to same addresses from laptop running XP.
I think failure message is from operating system.
Note that it always tries all my accounts and then records failure - I have an event log if anyone can decipher it.