Domainkeys is a method adopted recently by various service provider to make sure that mail is not spoofing.

You are receiving this error most probably as it suspect that mail is sent from another domain but its showing that its from different domain.

eg. .. may be mail is sent from "xyz.com" domain but you are seeing that mail is from "abc.com" so this error is there.

If you can post full header info here then I can tell you bit more!