Mail to <postmaster> at this host is still supposed to work.
Say what? That's only the case if the host runs an SMTP server,
in which case it is entirely logical to require support of a
postmaster address. It is entirely possible for a host to have
an SMTP client and no server.
For pure clients I'd look into RFC 4409, 4954, and 5068. When
the server in question is sure that it's not talking with one
of its own users and acting as MSA, where clients might have
funny names like "oemcomputer" or "xyzzy.invalid", an MX could
assume that an unknown stranger talking to it is an SMTP relay.
And for SMTP relays 2821bis 4.5.1 says "postmaster MUST work".
2821bis 220.127.116.11 does not help to decide if the other side is a
client or relay. In the <tm> real SMTP </tm> that might have
been obvious in the reverse-path, more than one hop is a relay.
postmaster is only required for servers that perform delivery
or relay functions. I presume that's so a stub server that
always returns 5xy errors isn't required to accept postmaster
That point is explicitly mentioned in 4.5.1, yes. It is kind
of obvious that MTAs always saying 554 instead of 220 cannot
accept mail to <postmaster>, nevertheless 2821bis mentions it.
But for an MX tormented by a client out of control a mail to
<postmaster> asking to fix whatever is broken can make sense,
if just blocking this client doesn't help or is undesirable.
I couldn't tell without cheating (= looking into 2821bis)
if that's MUSTard, SHOULD, or between the lines.
Well, now you forced me to check it.
If you believe otherwise you'll need to cite the relevant
sections of 2821bis to support your point. I couldn't find
anything along the lines of what you appear to be claiming
From the POV of an MX talking to a client that definitely is
not "one of ours", i.e. an unknown stranger, and that does
not use RFC 821 reverse-paths, assuming that "direct-to-MX"
is bad, it can be only a relay, and for relays it's a MUST.
Please don't tell me that this assumption is unjustified, I
know that some MXs still tolerate it when I try direct-to-MX
for reasons like "my provider won't let me report spam to SC"
or to check what RFC-I listable problems a domain might have,
after it somehow attracted my attention in a negative way.
If there is no way to decide what is a relay all MUSTard in
2821bis talking about relays would be a waste of time, so
how do you do this ?