Question How does Plesk Linux configure Postfix to generate the Authentication-Results header

[TimReeves]

I've been having problems using PHPMailer with attachments and DKIM, receiving instances are saying the DKIM signature is invalid, the body has been modified (without attachments the dkim signature is valid). So I wanted to check out the differences in the canonicalisation algorithms, which is probably where the problem lies. But for the life of me I can't find out how Plesk configures Postfix to test DKIM signatures (or SPF) on incoming mail (come to that, nor how it generates the signature on outgoing mails).

I've found various articles explaining how to manually add openDKIM or various Milters (mail filters) to Postfix, but none of them refer to Plesk, and I have not found any of the elements which are described in those articles on my Plesk-managed servers.

Hence my question: How does Plesk Linux configure Postfix to generate the "Authentication-Results:" header?

Any hints at all much appreciated!

Thanks, Tim