Secondary MX Servers and User Validation

Popularity:

10

Simplicity:

s

Impact:

4

Risk Rating:

6

Secondary mail servers, with higher preference (hence lower priority) in your DNS MX record, should be used only in case the primary server is down; however, you may find that regardless of their secondary status, they still get a considerable amount of traffic. The reason is simple: Spammers count on the fact that you are less likely to implement heavy filtering on your secondary mail server (which has the same relay rules as your master).

This assumption is very accurate. Secondary servers—which are sometimes offsite, under the control of other organizations, and possibly part of an MX exchange informal agreement—perform storage and forwarding of relayed emails without checking the address validity. This makes them very spammer-friendly since you are not going to be rejected over the SMTP connection, but SPAM emails are stored in the queue and will eventually bounce if the address is invalid, which is not a problem for spammers because they most likely spoofed the envelope sender, as we already mentioned. And since the envelope sender of SPAM messages is most likely forged and invalid, your bounces will stay in the queue until expiration time.

In addition, secondary mail servers rarely implement SPAM and antivirus filtering.

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