SpamAssassin Rule: MISSING_MIMEOLE
Standard description: Message has X-MSMail-Priority, but no X-MimeOLE
The message is pretending to be generated by a Microsoft email program which uses the extension header X-MSMail-Priority, but is missing the extension header X-MimeOLE which is characteristic of Microsoft email.
This suggests that the sender is using badly-written mailout software, rather than a genuine Microsoft email program.
The default scores for this rule can be found in the online list of tests.