4 - double spam tags
DMARC is being used by more and more companies to protect their senders from spoofing.
Its use requires proper authentication with SPF or DKIM and alignment of From / Mail-From domains.
For more information:
<dmarc> act on fraudulent email
Messages from senders with the _dmarc record,
if they are NOT authenticated, they are highlighted with two [ SPAM ] tags in the subject:
[ SPAM ] ... message subject ... [ SPAM ]
Messages without the _dmarc record, when both SPF and DKIM authentication fail,
are reported with a [suspicious] tag in the subject:
[suspicious] ... message subject ...