Consider performing a
SpamAssassin Rule: DATE_IN_PAST_24_48
Standard description: Date: is 24 to 48 hours before Received: date
Explanation
The Date: header is checked against the timestamps in the Received: header lines. If the clock generating the Date: header is accurate then the mail was generated 1 to 2 days before being received by one of the relays. This is unlikely in a normal mail client.
Further Info
The default scores for this rule can be found in the online list of tests.