Authentication-Results

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Writing

result = MailAuth.authenticate(raw, ip: ip, mail_from: from, helo: helo)
MailAuth.AuthenticationResults.format(result, "mx.example.com")
# "Authentication-Results: mx.example.com;\r\n dkim=pass header.d=..."

Add it at the top of the message before you deliver it. Remove any Authentication-Results: headers that claim to be from your own server first: a sender can write them.

Reading

MailAuth.AuthenticationResults.parse(raw, trusted: ["mx.google.com"])

This reads every Authentication-Results: header of a message, or one header. Each comes back with its authserv_id and trusted?, which is true only for the names you list. A header without a server name, as Outlook writes them, is never trusted.

Don't decide anything from an untrusted header. If you can, check DKIM and DMARC yourself with MailAuth.authenticate/2; that needs no trust in any header.