Mail delivery failed: returning message to sender errors are generally encountered when an intended recipient does not exist on the remote server.

You will normally receive a bounceback message with this error in the subject, and the body of the message should contain the original message that was attempting to be sent.

The study involved about 14.2 million messages with DKIM signatures, 5.33% of which failed to verify.

The messages came from 16,797 different domains, 10,968 (65%) of which had 100% verification rates and 2,899 of which failed consistently.

A spammer attempted to leave a comment on one of their posts, and the Word Press administrator of this site has setup their e-mail account as [email protected]

So that is the address used to send comment moderation requests to.

One of the main errors that I have seen is improper quoting of values in the key record with the backslash character.

To see how this might happen, consider a key record with the following contents: $ dig short selectorname._domainkey.txt"k=rsa; p=MHww DQYJKo ZIhvc NAQEBBQADaw Awa AJh ALl9ZT9er T0fc Vz YKgn Uki Ted Pn K/sy5Hw PXFD20zy GQe Slnt JWIr Iff5/QOh Jdh8df IZOm DK2Rv Yh VZSxt CB3g Bvlgo4Fw ERBWUNJpo QRf Q2xam Es IMSauj Ep4ym C3pw IDAQAB" Note the insertion of the backslash in the output from the command.

While there is a great deal of compatibility, one difference is the interpretation of the g= value, which is used as a constraint on the local part of the i= value (signing address) in the signature.

In Domain Keys, a key containing “g=;” (a null g= value) is interpreted to match any address; in DKIM, it is interpreted to match no address.

This isn’t really part of the DNS record, it’s just added in the output from the command.