DKIM: what it's not

An ESP twittered this past week about their new DKIM implementation going live. They were quite happy with themselves. Unfortunately, in their blog post, they mentioned 3 things that DKIM would provide for their customers, and got it wrong on all 3 points. Their confusion is something that a lot of people seem to get wrong about DKIM so I thought I would explain what was wrong.

  1. “[M]essages are affirmatively identified as coming from our servers…” DKIM isn’t necessary to affirmately identify mail as coming from a particular server or IP address. In fact, one of the major benefits of DKIM is that it allows sender reputation to be independent of IP address reputation.
  2. “Messages are more likely to be delivered to the inbox rather than the spam folder.” Not necessarily. The presence or absence of a valid DKIM signature is unlikely to increase inbox delivery on its own. Having a valid DKIM signature and a good reputation for that sender may result in better inbox delivery. The ISPs aren’t currently, are are unlikely to, offer preferential inbox delivery just on the basis of a DKIM signature.
  3. “Larger ISPs are heading towards requiring a DKIM signature on all incoming email. We are providing this feature now to avoid any issues in the future.” This is currently untrue and it is extremely unlikely that any ISPs will ever *require* a valid DKIM signature on all incoming email. The internet is just too large and too varied for ISPs to expect that all wanted mail will be DKIM signed.

DKIM is a way to authenticate email. Senders with good reputation will be able to take advantage of that reputation no matter what IP address they send mail from.
Senders should encourage ESPs and MTA vendors to implement DKIM signing sooner rather than later. However, DKIM signing alone will not improve delivery.

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AOL and DKIM

Yesterday, on an ESPC call, Mike Adkins of AOL announced upcoming changes to the AOL reputation system. As part of these changes, AOL will be checking DKIM on the inbound. Best estimates are that this will be deployed in the first half of 2009, possibly in Q1. This is something AOL has been hinting at for most of 2008.
As part of this, AOL has deployed an address where any sender can check the validity of a DKIM signature against the AOL DKIM implementation. To check a signature, send an email to any address at dkimtest.aol.com.
I have done a couple of tests, from a domain not signing with either DK or DKIM, from a domain signing with DK and from a domain signing with both DK and DKIM. In all cases, the mail is rejected by AOL. The specific rejection messages are different, however.
Unsighng domain: host dkimtest-d01.mx.aol.com[205.188.103.106] said: 554-ERROR: No DKIM header found 554 TRANSACTION FAILED (in reply to
end of DATA command)
DK signing domain: “205.188.103.106 failed after I sent the message.
Remote host said: 554-ERROR: No DKIM header found
554 TRANSACTION FAILED”
DK/DKIM signing domain: “We tried to delivery your message, but it was rejected by the recipient domain. We recommend contacting the other email provider for further information about the cause of this error. The error that the other server returned was: 554 554-PASS: DKIM authentication verified
554 TRANSACTION FAILED (state 18).”
As you can see, in all cases mail is rejected from that address. However, when there is a valid DKIM signature, the failure message is “554-PASS.”
As I have been recommending for months now, all senders should be planning to sign with DKIM early in 2009. AOL’s announcement that they will be using DKIM signatures as part of their reputation scoring system is just one more reason to do so.

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DKIM "i=" vs "d=" and Reputation

This really should be part seven of a twelve part series or some such as it deals with an aspect of DKIM that’s really important, but is way down in the details of implementation. (dkim.org is a reasonable place to start for a general overview of DKIM).
There’s an apparently endless thread on the DKIM-SSP spec development mailing list at the moment about the differences between two fields in a DKIM signature that could be used to tie a senders reputation to. Several ESP delivery folks asked me to explain what everyone was talking about, and this post is a first cut at that.
“i=” vs “d=”
There are two possible fields in a DKIM signature that could be used to identify the sender of a message, and so to tie a sender history and reputation record to. They are the so-called “i=” and “d=” field, from the syntax used to include them in the signature.

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DKIM implementation survey: prelim results

First off, I want to thank everyone who participated in the DKIM implementation survey. This week has been pretty hectic so far, so I haven’t had a chance to actually dig down into the data from the survey, but I thought I’d post some preliminary results.
The ESP survey had 45 respondents. 30% of those sent more than 15 million emails a month.
Of all the respondents: 40% are signing with Domain Keys, 51.1% are signing with DKIM.
Of all respondents: 79.5% are signing with Domain Keys and 78.8% are signing with DKIM to access services (whitelists or FBLs) provided by the ISPs.
50% of those not signing with Domain Keys are not doing so because customers have not requested it.  61% of those not signing with DKIM are not doing it because of technical difficulties with deployment.
The ISP survey had 16 respondents, with 37.5% handling less than 500,000 mailboxes and 18.8% handling more than 15 million mailboxes. 75% of respondents said they are not checking Domain Keys on inbound mail. 56% said they are not currently checking DKIM on inbound mail.
Only 10 ISPs answered the question if they plan to check either Domain Keys or DKIM.

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