Methods to reject mail to unknown recipients in Google Apps

Question

In Google Apps administration, under e-mail settings (Service Settings > Email), I have the option to specify a catch-all address or to discard the e-mail.

But, I'm wondering how I would configure rejection of the e-mail (i.e. 550 error) instead of simply discarding it, because I want people who send to an incorrect address to know they have sent to a wrong address.

What are methods for accomplishing this? Do I set a catch-all address and then monitor that mailbox with a Daemon that generates 550 rejection notices? Do I do something crazy with my MX records?

Answer 1

Google Apps already sends a 550 error if you don't have a catch-all address set.

Here is the results of a message I just sent to a invalid account on a Google Apps domain (example.org) from my gmail account. Names have been changed to protect the guilty. The domain is set to 'discard'.

Delivered-To: [email protected]
Received: by 10.216.45.139 with SMTP id p11cs30689web;
        Tue, 21 Sep 2010 12:05:45 -0700 (PDT)
Received: by 10.216.176.83 with SMTP id a61mr9625704wem.47.1285095944841;
        Tue, 21 Sep 2010 12:05:44 -0700 (PDT)
MIME-Version: 1.0
Return-Path: <>
Received: by 10.216.176.83 with SMTP id a61mr12854859wem.47; Tue, 21 Sep 2010
 12:05:44 -0700 (PDT)
From: Mail Delivery Subsystem <[email protected]>
To: [email protected]
X-Failed-Recipients: [email protected]
Subject: Delivery Status Notification (Failure)
Message-ID: <[email protected]>
Date: Tue, 21 Sep 2010 19:05:44 +0000
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

Delivery to the following recipient failed permanently:

     [email protected]

Technical details of permanent failure:=20
Google tried to deliver your message, but it was rejected by the recipient 
domain. We recommend contacting the other email provider for further inform
ation about the cause of this error. The error that the other server return
ed was: 550 550-5.1.1 The email account that you tried to reach does not ex
ist. Please try
550-5.1.1 double-checking the recipient's email address for typos or
550-5.1.1 unnecessary spaces. Learn more at                            
550 5.1.1 http://mail.google.com/support/bin/answer.py?answer=3D6596 p70si1
2864980weq.200 (state 14).

----- Original message -----

MIME-Version: 1.0
Received: by 10.216.176.83 with SMTP id a61mr9625683wem.47.1285095944055; T
ue,
 21 Sep 2010 12:05:44 -0700 (PDT)
Received: by 10.216.45.139 with HTTP; Tue, 21 Sep 2010 12:05:44 -0700 (PDT)
Date: Tue, 21 Sep 2010 12:05:44 -0700
Message-ID: <[email protected]
>
Subject: invalid user test
From: Chris Francy <[email protected]>
To: [email protected]
Content-Type: text/plain; charset=3DISO-8859-1

invalid user test

Answer 2

I use a catch-all account that has a Vacation responder.

No backscatter is produced, because incoming spam is filtered by Google. So the Vacation responder only replies to incorrectly addressed "real" messages.

The catch-all account also forwards a copy of all incoming mail to me, so I can monitor it.

Answer 3

The way Google Apps is configured is an increasingly common way of configuring mail. It is entirely possible for bad entities to perform what amounts to a directory attack by throwing commonly used usernames at a mail domain, and anything that doesn't produce a 550 error is considered to be a verified live address (we have a guy in the office who has "steve" as his account name. He gets more spam than you do). If you never return a 550, they can't know they found a bad one.

The utility of that is somewhat debatable in this era of IP-reputation based antispam systems, but it can matter to some.

Answer 4

Just create a catch-all address with an auto-responder. You can use the auto-responder to let people know they've sent mail to a user that doesn't exist.