David Woodhouse <dwmw2(_at_)infradead(_dot_)org> wrote:
... it doesn't necessarily matter _which_ name as long as you can
be sure the sending party is really supposed to be using that name.
Which is why I don't agree with you that they're entirely orthogonal.
They're different, it's true; but what they achieve in the long run is
basically the same.
What I mean by "orthogonal" is that SMTP makes no requirements on
the relationship between the values given in EHLO and MAIL FROM. They
may use similar domain names, or they may use completely different
domain names. So the fields are "orthogonal", because there is no
dependency relationship between them.
Now, *validation* of those fields can use correlations between their
values to make decisions:
e.g. EHLO mta.example.com
MAIL FROM <user(_at_)example(_dot_)com>
Since the recipient discovers for himself that the values of the two
fields are correlated, he may choose to "merge" the field validation.
e.g. "example.com says this IP really is mta.example.com, and is
permitted to act as an SMTP client, so I'll assume that I don't have
to ask example.com again about the use of it's name in the MAIL FROM
If the two fields had different domain names, then the validation of
those fields would have to be independent.