The Domain White List (DWL) a VBR (vouch-by-reference) domain whitelist designed to automate DKIM certification. VBR is designed to be used in conjunction with DKIM checking on the receiving server. After the receiving server checks the messages's DKIM signature, it should then check the DKIM signing domain against the DWL.
For VBR implementation see: http://en.wikipedia.org/wiki/Vouch_by_Reference and in particular RFC 5518 for implementation.
Note: The Domain White List (DWL) should not be used to whitelist messages based on domains seen in email bodies or headers. Only the d= domain identity in a valid DKIM signature provides a usable domain for the DWL. Domain names or URLs in other contexts are easy to forge and are not reliable identifiers.