Dmarc

  * https://www.dnswl.org
  * https://dkimvalidator.com/
  * http://multirbl.valli.org
  * https://dmarc.org/resources/deployment-tools/
  * https://toolbox.googleapps.com/apps/checkmx
  * https://postmaster.google.com/u/0/managedomains

  * autoreply@dmarctest.org
  * check-auth@verifier.port25.com
  * mailtest@unlocktheinbox.com

To check that the mail server works:

$ sendmail -tv -F example -f example@ircnow.org < ~/samplemail

Inside ~/samplemail:

From: example <example@ircnow.org>
To: mailtest@unlocktheinbox.com
Subject: Alpha Bravo
MIME-Version: 1.0 
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline

Charlie Delta Echo Foxtrot
<<< 220 mail.ircnow.org ESMTP OpenSMTPD
>>> EHLO localhost
<<< 250-mail.ircnow.org Hello localhost [local], pleased to meet you
<<< 250-8BITMIME
<<< 250-ENHANCEDSTATUSCODES
<<< 250-SIZE 36700160
<<< 250 HELP
>>> MAIL FROM:<example@ircnow.org>  
<<< 250 2.0.0 Ok
>>> RCPT TO:<mailtest@unlocktheinbox.com> 
<<< 250 2.1.5 Destination address valid: Recipient ok
>>> DATA
<<< 354 Enter mail, end with "." on a line by itself
>>> .
<<< 250 2.0.0 66f4c965 Message accepted for delivery
>>> QUIT
<<< 221 2.0.0 Bye