What about RFC 1123 where it states:
5.2.5 HELO Command: RFC-821 Section 3.5
The sender-SMTP MUST ensure that the <domain> parameter in a
HELO command is a valid principal host domain name for the
client host. As a result, the receiver-SMTP will not have to
perform MX resolution on this name in order to validate the
HELO parameter.
The HELO receiver MAY verify that the HELO parameter really
corresponds to the IP address of the sender. However, the
receiver MUST NOT refuse to accept a message, even if the
sender's HELO command fails verification.
It is very important to keep e-mail policy separate from e-mail MTA
correctness. If I violate the above in my MTA, then I may not be
compliant with RFC 1123, and my MTA is not "correct"ly implemented.
However, as an e-mail admin, I have an
absolute right to refuse or accept any mail I want to, on any basis I choose.
The MTA is governed by RFC 1123, and it needs to give the admin
the choice of accepting the mail.
SPF is a tool that can be used by the admin to determine which
e-mail to accept based on policy.
-dgl-
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