🚀 go-pugleaf

By an unreplyable address, I mean an address to which mail cannot be
sent, but which is found in the From: or Reply-To: header line of a
mail or news message.  Common and obvious examples are simply
malformed non-addresses (e.g. joe@undotted-name or site!joe or
joe%site); but more subtle ones include well-formed addresses that are
continually inaccessible, such as those for which valid MX records are
available but which direct the SMTP socket toward an un"connected"
network (e.g. pentagon-ai.army.mil).

The only explicit reference I can find to unreplyable From: lines is
in RFC1123, section 5.3.7(D) of the discussion of gatewaying between
the Internet and other mail systems, where we find that `... all
addresses ... must be effective and useful for sending replies.'
(Note that 5.3.7 discusses mail that crosses mail network boundaries,
but not Internet-only mail!)

This is at least a bit more explicit than RFC822, section 4.4.1 of the
discussion of the From: field, where we see that `... addresses in the
"From" field must be machine-usable (addr-specs) ...'  One might
broadly interpret "machine-usable" to mean "accessible via SMTP and
the DNS", but that seems to be stretching the interpretation a bit
further than most Talmudic scholars of the RFCs would prefer.

I think that most mail users and administrators would agree that
unreplyable From: or Reply-To: lines are Wrong.  Why is there no
stronger and more specific prohibition?