I notice a lot of spammy looking e-mails from suspicious sources. They are not real spam but are unsolicited. You can identify them by containing these headers:
X-Mailer-LID: 5 List-Unsubscribe: <http://example.com/unsubscribe.php?M=605065&C=b9fd2d5e30ef6ba9f399d0536f293b69&L=5&N=4> X-Mailer-RecptId: 605065 X-Mailer-SID: 4 X-Mailer-Sent-By: 1
Does anyone know the software that generates those mails? They are sent out afterwards via real MTAs like Postfix or Exim.
If one could verify the software you can use that information for spam filtering.