[Mailman-Users] Google mail servers reply "Multiple destination domains per transaction is unsupported "

Joseph Brennan brennan at columbia.edu
Sat Mar 9 14:53:09 CET 2013

"Stephen J. Turnbull" <stephen at xemacs.org> wrote:

> I bet they think it's an anti-spam measure.  None of the big services
> likes to talk much about that.

That's what frontline helpdesk told me, but no one could explain how
it reduces spam. It beats me why, say, 6 recipients at one domain is
less spammy than 6 recipients at two domains. The reverse might even
even be true, since spammers seem to sort by domain not by mx server.

>  > but they confirmed it is not considered a bug, it was done
>  > deliberately.
> Can you confirm that this is happening at your outgoing MTA (ie, the
> first Google MX you encounter when submitting as a Google customer)?

We run our own smtp servers. They determine where each user's mail
goes. Some users are on Google Apps, so we re-send their mail to
Google's MX address. That's when the fun begins.

What I demonstrated is what happens when any host anywhere connects
to Google's MX address and tries to send to addresses at two domains
both hosted by Google. These are the hosts:

$ host -t mx gmail.com
gmail.com mail is handled by 5 gmail-smtp-in.l.google.com.
gmail.com mail is handled by 30 alt3.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 10 alt1.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 40 alt4.gmail-smtp-in.l.google.com.
gmail.com mail is handled by 20 alt2.gmail-smtp-in.l.google.com.

> I'm just guessing, but in practice is this issue restricted to mailing
> lists and other software that automatically mails to several users?
> Did they perhaps say "you should be using the submission protocol
> (port 587)"

No, it was just a non-Google host trying to send mail to two addresses
hosted on Google. That's all it takes to get that response.

> The reason I ask is that surely academic users regularly send mail
> personally to recipients at multiple domains.

Of course! I used columbia.edu and barnard.edu as an example because
the two schools are very closely affiliated. We first noticed this when
an undergrad club president wanted to know why the members at one school
got their mail right away and the others were delayed about 20 minutes.
The answer is more or less "because Google is weird".

Joseph Brennan
Manager, Email and Systems Applications
Columbia University Information Technology

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