4 - 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 ...