How not to be a spammer [3]: the DNS is your friend

How can legitimate direct marketers get their messages through more reliably? How can they avoid being branded as a "spammer" by over-enthusiastic spam vigilanties?

We wrote about this problem before. See [1] and [2]. This is the first of an irregular series of blog posts where we’ll examine some additional ideas.

The Domain Name System is your friend.

People often think of the DNS as just a white pages for the Internet. Give it a domain name like mail.example.com and it’ll give you a number like 1.2.3.4—its IP address. But don’t forget that DNS can work the other way too.

Make sure that your mail server not only has an entry mapping its domain name to its IP address, but also one that maps its IP address back to its domain name (a PTR record). If your mail server sends mail on behalf of more than one domain, make sure you have a reverse-mapping entry for each domain. Where possible, do the same with the sending client, if it submits mail via SMTP.

Why’s this important? Many anti-spam tools look at the sender’s domain and compare it with the sending machine(s), in order to check for a forged sender. The latest example is IBM’s FairUCE project.

Keep an eye on the blog for more tips about how not to be a spammer soon.

[Edits: to fix a broken link and mention PTR records.]

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