Hi,
I was facing the same "provider considers my emails spam" so I included all signing mechanisms (DomainKeys via Plesk and DKIM via opendkim) and I got:
The Port25 Solutions, Inc. team
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Summary of Results...
Same issue here.
I've been trying to solve it but I couldn't, so I decided to use the same workaround (remove psa-pc-remote from non_smtpd_milters) for the moment...
Thanks timber.
EDIT: if I do that I lose DKIM signature, so I have decided to keep DKIM and lose DomainKeys using milter_protocol=2