SpamAssassin Rule: DKIM_ADSP_DISCARD
Standard description: No valid author signature, domain signs all mail and suggests discarding the rest
Explanation
The sender's domain says that it uses DKIM (http://www.dkim.org/) on all email, but no valid signature was found. The sender's domain policy suggests discarding this message.
That suggests that the message might not have originated with the purported sender.
Further Info
The default scores for this rule can be found in the online list of tests.