bind config:
_domainkey.mailer.mydomain.com                      TXT     "t=y; o=-"
myselector._domainkey.mailer.mydomain.com        TXT     "k=rsa; t=y; p=MY_PUBLIC_KEY"


exim log message:
2009-06-04 19:46:23 1MCHxG-0003jR-3H DK: message signed using a=rsa-sha1; q=dns; c=nofws; s=myselector; d=mailer.mydomain.com; h=Received:From:To:Subject:Message-Id:Sender:Date;