Consider performing a
SpamAssassin Rule: DKIM_POLICY_SIGNALL
Standard description: Domain Keys Identified Mail: policy says domain signs all mails
Explanation
The sender's domain says that it uses DKIM (http://www.dkim.org/) on all email, but no valid signature was found.
That suggests that the message did not in fact originate with the purported sender.
Further Info
The default scores for this rule can be found in the online list of tests.