Eric A. Hall writes:
Secondarily, some of the recipient-specific
fields can't be implemented as shared header fields
Have one field for each recipient, naming the recipient:
Notice-Requested-Upon-Delivery-To: ehall(_at_)ehsco(_dot_)com
Notices to the envelope sender, of course. This is not rocket science.
Handling DSN as an SMTP extension was a serious error. DSN was designed
to achieve something users don't care about, namely guaranteeing an ``I
dunno'' response when no other response was forthcoming, at the expense
of what the users actually wanted, namely return receipts.
By the way, the guarantee was destroyed by the ``noreceipts'' option in
sendmail 8.6.10 through 8.9.3: sendmail accepted DSN requests and then
threw them away.
---D. J. Bernstein, Associate Professor, Department of Mathematics,
Statistics, and Computer Science, University of Illinois at Chicago