Consider performing a
SpamAssassin Rule: RATWARE_OUTLOOK_NONAME
Standard description: Bulk email fingerprint (Outlook no name) found
Explanation
The Message-Id in the header seems to be generated using a scheme found in Microsoft software, but is missing a specific identifier to confirm that this software was used.
Further Info
The default scores for this rule can be found in the online list of tests.
See also: Rules/RATWARE_MS_HASH