MAIL FROM is <postmaster(_at_)example(_dot_)com> instead of <>.
Ugh.
I confirm that IIS SMTP / Microsoft Exchange uses the NULL envelope
sender for DSN's.
(Even worse, they don't
reject the mail they are rejecting, but accept it, then send it to me wrapped
as a non-DSN claiming to be a DSN saying "we don't want it".)
Well, the IIS SMTP server is just an SMTP server. It does not validate
the recipients, it only knows the list of domains and accepts email
for any recipient addresses in these domains.
The IIS SMTP server is also used by Exchange 2000/2003, which hooks
specific events of the SMTP Service. In fact, Exchange 2000 accepts
the recipient first at protocol level and generate a DSN later, if the
recipient was invalid. Exchange 2003 enable you to turn on
protocol-level recipient validation.
It's only matter of choice. You don't do any damage by generating the
DSN later (note that it's the same effect like in the case of a
multi-stage relay), but you do impose your server to successful DHA's
if you enable the protocol level validation.
Peter