Symptoms

1. Mail delivery fails with error 421 in /usr/local/psa/var/log/maillog file:

       postfix/qmgr[26602]: E8A30280756: from=<test@domain.tld>, size=18764, nrcpt=1 (queue active)
       postfix/smtp[16849]: E8A30280756: host refused to talk to me: 421 4.7.1 : (DNS:NR) http://postmaster.info.aol.com/errors/421dnsnr.html
       postfix/smtp[16849]: E8A30280756: to=<user@aol.com>, delay=55016, delays=55014/0.02/2.3/0, dsn=4.7.1, status=deferred (host testtld-01.mx.aol.com refused to talk to me: 421 4.7.1 : (DNS:NR) http://postmaster.info.aol.com/errors/421dnsnr.html)
   

2. Reverse DNS zones are not configured under Services > DNS Zones > DNS tab > Reverse DNS Zones

Cause

Messages are not delivered due to the Reverse DNS failure.

Resolution

Make sure that you can connect to mail server via telnet:

    # telnet mailin-01.mx.aol.com. 25
    Trying 64.12.88.132...
    Connected to mailin-01.mx.aol.com..
    Escape character is '^]'.
    220-mtaig-mcb01.mx.aol.com ESMTP Internet Inbound
    220-AOL and its affiliated companies do not
    220-authorize the use of its proprietary computers and computer
    220-networks to accept, transmit, or distribute unsolicited bulk
    220-e-mail sent from the internet.
    220-Effective immediately:
    220-AOL may no longer accept connections from IP addresses
    220 which no do not have reverse-DNS (PTR records) assigned.

According to documentation PPA does not automatically create reverse DNS zones for the registered IP network addresses.

Manually create reverse DNS zones for all of your IP network addresses in Services > DNS Zones > DNS tab > Reverse DNS Zones.

Once a reverse DNS zone is created for a certain network, PPA will automatically add to it corresponding PTR records for all newly created domains.