Prevent Spam Mail From Your Own Domain in Exchange 2007
One of the biggest bug-bears with spam is the spam that comes from (or supposedly comes from) random_username@yourdomain.com or even your_username@yourdomain.com. This is known as spoofed mail and is a common technique that spammers use to try to get mail past Anti-Spam software.
From the Anti-Spam logs on my own server in the last 24-hours, I have received 1,974 emails (out of 17,432 in total) where the sender domain matched the recipient domain. This is about 11.3% of all mail that hit my server, so it is a relatively large problem. Factor that up to a year’s worth of mail and you get 720,510 a year.
To prevent this from happening, you simply need to remove a specific permission that allows anonymous senders to use your internal domain names in the Mail From section of an email. If anyone tries to do this (anonymous users only) they will receive a “550 5.7.1 Client does not have permissions to send as this sender” message.
The syntax to remove the permission should be entered as follows in the Exchange Management Console:
Get-ReceiveConnector “My Internet Receive Connector” | Get-ADPermission -user “NT AUTHORITY\Anonymous Logon” | where {$_.ExtendedRights -like “ms-exch-smtp-accept-authoritative-domain-sender”} | Remove-ADPermission
(You need to change the “My Internet Receive Connector” part in the above syntax)
Having run this command successfully, test using Telnet to your mail server from an external computer and see what happens if you try to send mail as one of your internal domain names. You should receive the 550 5.7.1 Message.
N.B. To put the permission back (in case you need to), please run the following:
Get-ReceiveConnector “My Internet Receive Connector” | Get-ADPermission -user “NT AUTHORITY\Anonymous Logon” | where {$_.ExtendedRights -like “ms-exch-smtp-accept-authoritative-domain-sender”} | Add-ADPermission
If you have internal photocopiers and other hardware that needs to relay via your Exchange 2007 server and you cannot configure them with a username / password, then removing the above permissions will prevent you from relaying and will cause you problems.