Re: Correct value for EHLO when submitting mail

* Jan Kundrát <[email protected]>:
> when a MUA submits e-mail over SMTP, what value shall be used in the
> HELO/EHLO greetings? Shall I just stick with "localhost", or trust
> the OS-reported hostname, if any, or shall I use the IP address of
> my local endpoint?
> 
> I see problems with all of these, and apparently Thunderbird went
> through a nice discussion on this matter [1]. I didn't find any
> answer about that in RFC 6409, unfortunately, and RFC 5321's
> discussion [2] about this topic leaves me without a definitive
> answer, either -- the MUA will have zero idea about the "primary
> host name" and its relation to what OS returns, so I'm inclinded to
> interpret this as a suggestion to use address literals, but then
> I've heard about spam filters penalizing such messages [1]...
> 
> What is the current best practice?

Dunno if this is common best practice, but here's my rationale:

In MTA to MTA communication on port 25 it makes sense to check for reasonable
HELO hostnames:

- not your own
- not from a private network you can't connect back (bounce)

In MUA to MTA communication on port 587 it doesn't make sense to check for or
enforce HELO hostnames:

- the underlying OS often does not return a valid HELO hostname (not a FQDN)
- the identity sent during AUTH creates the trust required to accept and
  transport the message

p@rick

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