SPF: It’s Not Just a Good Idea

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Last week, Microsoft announced it will check for SPF records in all email coming into its Hotmail service, beginning October 1, 2004.

That’s all well and good, you may say, but what’s "SPF"?

Sender Policy Framework is one of several email authentication schemes that have been vying for attention and adoption during the past few months. Others include Yahoo’s Domain Keys and ePrivacy Group’s Trusted Email Open Standard (TEOS).

According to the SPF Web site, SPF:

Fights email address forgery and makes it easier to identify spams, worms, and viruses. Domain owners identify sending mail servers in DNS. SMTP receivers verify the envelope sender address against this information, and can distinguish legitimate mail from spam before any message data is transmitted.

What does this mean? On the most basic level, SPF allows mail server administrators to assert "the mail server at this IP address sends email from x.com, y.com, and z.com." Thus, when an email arrives at the border of the receiving system, that system looks up, via DNS, to see whether an SPF record is associated with that IP address.

As the SPF site explains it:

SPF makes it easy for a domain, whether it’s an ISP, a business, a school or a vanity domain, to say, "I only send mail from these machines. If any other machine claims that I’m sending mail from there, they’re lying...."

And that’s it! SPF aims to prevent spammers from ruining other people’s reputations. If they want to send spam, they should at least do it under their own name.

SPF isn’t a great way to identify and stop spam in and of itself. But it is a fabulous way to authenticate the identity of email senders.

Authentication is important because it helps prevent transmission of email containing forged sender information, as with phishing and, indeed, most spam. Phishing is the act of sending email with an intentionally forged sender address, usually a well-known company. The phisher tries to get the target to follow a link and reveal sensitive information, such as a password or credit card information.

Microsoft’s announcement means that beginning in October, the company will check inbound email’s source IP address for an SPF record. In other words, Microsoft will check whether the person responsible for that outbound mail server has published an SFP record. E-mail that passes the SPF test will be passed through for delivery. E-mail that doesn’t pass this simple test must go through additional hoops before it can be delivered to the inbox.

Bottom line: If you haven’t already done so, publish an SPF record. It’s fairly easy. The good folks at SPF even offer a setup wizard. Give it a whirl!

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