[Mimedefang] Non-routable addresses in HELO

Jan-Pieter Cornet johnpc at xs4all.nl
Tue Jul 11 07:18:21 EDT 2006

On Tue, Jul 11, 2006 at 12:37:44PM +0200, Steffen Kaiser wrote:
> On Tue, 11 Jul 2006, Jan-Pieter Cornet wrote:
> >So, based on this latter paragraph, it is entirely OK to reject
> >after "EHLO", since that is not a properly formatted EHLO
> >argument. RFC821 has similar restrictions on the HELO argument,
> >which should be a "domain" (no IP allowed).
> RFC821 is funny, because it allows this:
> [].[].#123445.DummDiDumm
[...BNF explantion...]

But there are additional limitations in the text of RFC821, to

from section 3.5 Opening and Closing:

      HELO <SP> <domain> <CRLF>

from section 3.7 Domains:

      Whenever domain names are used in SMTP only the official names are
      used, the use of nicknames or aliases is not allowed.

And from the glossary:


      The hierarchially structured global character string address of a
      host computer in the mail system.

And it is indeed fun to read, because the DNS system has only just
been invented when this was written, and that clearly shows :)
> BTW: That somebody knows what the "#" <number>  element is used for?

That knowledge might have been lost together with Jon Postel. I've
never seen it used.

Jan-Pieter Cornet <johnpc at xs4all.nl>
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