HOW TO: Prevent annoying spam from your own domain

One of the more annoying types of spam is the one that seems to be coming from your own domain; or worse— from your own email address! Of course, users from your own domain don't generally spam each other— unless you're using one of the free web-based email services. And most of us don't spam ourselves.

Obviously, this is coming from a spammer who has spoofed your email address, or that of someone else from your domain. Unfortunately, SMTP— the protocol that allows mail clients and servers to exchange email, allows headers to be spoofed easily.

In Exchange Server 2007, Accepted Domains tell Exchange which domains to accept email for. If a domain - e12labs.com in this example, exists as an Accepted Domain, there is no reason external senders should use that domain in the MAIL or FROM headers.

You may have remote POP3/IMAP4 users who use SMTP to send mail. However, such sessions should be authenticated, and preferably use a separate Receive Connector.

Thanks to the extensive Transport Permissions model in Exchange 2007, we can easily prevent such spam. Receive Connectors have the ms-exch-smtp-accept-authoritative-domain-sender permission which dictates whether an Accepted Domain can be used in the MAIL or FROM headers. External/internet hosts submit mail to your server without authentication, as anonymous senders. To prevent anonymous senders from sending mail using your domain(s), we need to remove the ms-exch-smtp-accept-authoritative-domain-sender permission assigned to them.

Use the following command to remove the ms-exch-smtp-accept-authoritative-domain-sender permission from NT Authority\Anonymous Logon on internet-facing Receive Connector(s):

Get-ReceiveConnector "My Internet ReceiveConnector" | Get-ADPermission -user "NT AUTHORITY\Anonymous Logon" | where {$_.ExtendedRights -like "ms-exch-smtp-accept-authoritative-domain-sender"} | Remove-ADPermission

Once this permission is removed, when anonymous senders try to submit mail using your Accepted Domain(s), here's how the SMTP conversation goes:

220 E12Postcard.e12labs.com Microsoft ESMTP MAIL Service ready at Wed, 3 Sep 2008 06:22:43 -0700
helo
250 E12Postcard.e12labs.com Hello [172.31.0.170]
mail from:jadams@e12labs.com
550 5.7.1 Client does not have permissions to send as this sender

Exchange stopped spoofing of P1/envelope headers. Let's continue the session and try to spoof the P2 headers (the ones in the DATA part of the message) — maybe that'll work!

mail from:someone@someotherdomain.com
250 2.1.0 Sender OK
rcpt to:jadams@e12labs.com
250 2.1.5 Recipient OK
data
354 Start mail input; end with .
from:jadams@e12labs.com
subject: Header spoofing

This is how we spoof headers, spoof headers.

.
550 5.7.1 Client does not have permissions to send as this sender
quit
221 2.0.0 Service closing transmission channel

As you can see, removing the ms-exch-smtp-accept-authoritative-domain-sender permission stops spoofing of your domains in both envelope (P1) and message (P2) headers.

When not to remove the permission?
Is there a scenario where one should not remove the ms-exch-smtp-accept-authoritative-domain-sender permission from NT Authority\Anonymous Logon? Yes, on Receive Connectors used by internal or trusted SMTP hosts (such as copiers/scanners and application servers) that submit mail without authentication.

But you do have these internal/trusted hosts submitting to a separate Receive Connector, don't you?

Related posts:

• Exchange Server 2007: How To Allow Relaying
  
• Removing internal host names and IP addresses from message headers
  
• Header Firewall: Why spammers can't insert fake SCL (and other Exchange Organization) X headers