* 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

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

  * 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