I have only one thing to add to Jim's thorough analysis: if you don't
want to have to put up with broken mail software, you can SERVE off the
local postmaster address to prevent messages with undetermined origins
from being distributed. I can't make this decision for everyone because
some sites may have valid reasons to accept such messages (examples: the
PC the Director of Computing uses to dial up from home behaves this way
;-) ).
Eric