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.