See https://ademcirak.com/configure-server-for-sending-signed-email-dkim-spf-and-dmarc.html.

DKIM requires some knowledge about generating certificates. However, PHPMailer’s DKIM examples should help tremendously: https://github.com/PHPMailer/PHPMailer/tree/master/examples

DMARC is just a record you add to your DNS after you’ve done SPF and DKIM. This wizard I found can help create this: https://dmarcian.com/dmarc-record-wizard/

I also found http://www.appmaildev.com/en/dkim which will help you verify DKIM, SPF, and DMARC.

Google’s answer here gives some other tips to take a look at: https://support.google.com/mail/answer/81126. I haven’t tried out Google’s Postmaster tools, but Gmail itself does provide indication if DKIM and SPF pass.

Hello, all

You can make sure that your mailing list is fully opted-in and that you’re using SMTP authentication.

Also to test the mail score of your emails you can use a website like:

https://www.mail-tester.com/

and then send a valid test - an exact email with a valid - subject, message body, full signatures and attachments.

Regards,
Alex