DKIM and Domain Keys overkill?

Question

I have noticed that a number of emails I get have a DKIM header. Some emails I get have the DomainKey header. And yet some other emails include both headers. Is there any reason to have an email backed by both DKIM AND DomainKeys or is one or the other sufficient?

If using both of these on each email is overkill, assuming you have DomainKeys already working is there any benefit to replacing it with DKIM?

As is the case with everyone asking these types of questions I want to make sure that I accommodate to the most number of target domains as possible.

Answer 1

DomainKey is the older version of DKIM (RFC4870). DKIM is based on RFC4871, and the way to do it. Using both versions together will allow old implementations to verify the signature.