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