[geeks] A puzzling mail error

[der Mouse]

> I believe it's trying to tell you that pond is HELOing as
> [127.0.0.1].

I agree.

> Aside: "HELO 1.2.3.4" is broken: missing brackets.

Well...I prefer to think of it as broken because no such domain exists
in the DNS (.4 is not an existing TLD); syntactically, it's a perfectly
good domainist HELO.

> "HELO mail" is broken: not a FQDN.

Why not?  It's nonexistent, because no TLD "mail" exists, but that
doesn't make it any less a FQDN.

However, it wouldn't be valid even if such a TLD did exist; despite the
language in 2.3.5 that appears to permit one-label names, the syntax in
4.1.2 makes it clear that a "Domain" (such as appears in a domain-style
HELO argument) must include at least one dot.  (Section references are
to RFC2821.)

> "HELO fubar.example.com" is broken if fubar.example.com does not
> exist in DNS.

Yes.

> "HELO [192.168.0.1]" is broken if received on an external interface
> that shouldn't be talking to anything in RFC 1918 space.

Is it?  I can't find anything that says the address embedded in an
address literal has to bear any particular relationship to the address
of any machine anywhere, much less either of the machines participating
in the SMTP conversation.  Did I miss something?

Of course, it may indicate problems in a pragmatic sense, but that's
hardly the same thing.  Indeed, I once arranged for a friend to send
mail through my mailserver via an ssh tunnel; the machine invariably
HELOed as an address-literal containing a 1918-private address.  (Of
course, this particular case was covered by private arrangement, but
still.)

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