We've been noticing a large percentage of recipients never receiving Mailing List emails in their Inbox when sent from SmarterMail . Looking into the problem we were told by Google that reason they are flagged as Junk Mail, despite passing SPF, DKIM, DMARC is that they do not contain a List-Unsubscribe Header (we do have the Unsubscribe variable link in the Footer)
To my knowledge there is no way with SmarterMail to add a custom Header to MailingLists that is within the DKIM signed portion of the headers and message.
Could there be a way to add the List-Unsubscribe Header according to RFC 2369 in the SmarterMail Mailing Lists?