[rfc-i] SMTP-HELO clarification

rfc at dvl.sh-ks.net rfc at dvl.sh-ks.net
Mon Sep 18 07:12:48 PDT 2017

we've problems with some MTA-admins, which interpret the rfc in
another way, than we do.

maybe someone could help to clarify.

rfc 1123, section 5.2.5, page 50f says, that the receiver may verify
the HELO-parameter, but MUST NOT refuse acceptance on failure.

rfc 2821 (which updates, but not obsoletes rfc 1123), section,
page 29 describes, how HELO has to be used and how the parameter
should look like, but does not explicitly refer to rfc 1123, how the
parameter-content has to be interpreted.

as we understand the rfc-mechanisms, rfc 2821 don't need to refer
explicitly to certain parts of rfc 1123, except when it makes a change
or limit to that rfc.

unfortunately some MTA-admins doesn't match this and say, that rfc
1123 has been updated by rfc 2821 and doesn't repeat the relevant
section, so this won't be valid any more.

(the result is, they reject messages because of HELO/hostname-mismatches)

so, who is right on interpretation of the RFCs?

and: is there any RFC, which defines, that the FQDN-parameter of the HELO
1. *MUST* match the reverse-mapping of the used IP-address?
2. *MUST* resolve to the used IP-address?

rfc 2821 only say, that if no FQDN is available, an address-literal
should be used.


best regards


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