> How often do you read the To header. Keep in mind that in email the To header does not have to include the intended recipient.

Perhaps if the address at which you received the email does not match any which are covered by the DKIM signature then your client could warn you about the potential for foul play?

I don't think that would be a good idea. The false positive rate would be too high and i don't think forging the to header is useful enough to phishing to make it worth it.