Spammers using my backup mx service to bypass spam filtering

Posted on 2007-08-01
Last Modified: 2008-05-05
I am Running Exchange Server 2003 SP2. I am using a backup mx service my ISP offers, please see my mx records below. The problem I am having is spammers are sending their email through the backup mx server instead of directly to my server, so when the spam finally ends up getting forwarded from the backup mx server to my email server it seems that both RBL checks and domain validation checks are failing on these spam messages. For now I have blocked all email from the backup mx server by using connection filtering in exchange and blocking the mx servers ip address. What can I do to defeat this work around that the spammers are using to bypass some of my spam filtering in exchange?

nslookup -q=mx    MX preference = 20, mail exchanger =    MX preference = 10, mail exchanger =
Question by:DMS-X
    LVL 104

    Expert Comment

    That is a common spammers method. The reason they are doing it is because it usually bypasses the checks.
    If your antispam strategy relies on connection filtering controls then you only have one option - remove the second MX record.

    LVL 1

    Author Comment

    What about if I were to smart host my email server through my isp instead? How would this effect my spam filtering methods in exchange?
    LVL 104

    Accepted Solution

    A smart host is for outbound email, so any changes in that aspect would have no effect on inbound email.

    LVL 12

    Assisted Solution

    Always..a spam filter is needed per MX record...
    If you remove your secondary record, you could use a router and assign a cost on the routes between the two boxes. Or, an smtp server at the gateway to receive inbound mail.

    Depending on the time it takes to update your MX record on your public dns server, you could make the change there manually in the event the primary mail server is down.

    It is not that big of a deal for inbound mail because most mail servers will queue the message if your server is down.


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