I use SmarterMail 15.7.  I am receiving error messages when sending to att.com or comcast.com addresses similar to these:

Reason: Remote host said: 553 5.3.0 alpd680 DNSBL:RBL 521< 50.xxx.xxx.xxx >_is_blocked.For assistance forward this error to abuse_rbl@abuse-att.net

Reason: Remote host said: 554 resimta-ch2-01v.sys.comcast.net resimta-ch2-14v.sys.comcast.net 2002:32cd:4734::32cd:4734 Comcast requires that all mail servers must have a PTR record with a valid Reverse DNS entry. Currently your mail server does not fill that requirement. For more information, refer to: http://postmaster.comcast.net/smtp-error-codes.php#554

I host multiple domains and the error messages come back when mail is sent from certain domains, but not all of them.  My server is configured with two fixed IP 4 addresses.  My internet service is provided by Comcast.  The primary IP address on the server is 50.xxx.xxx.52 with a secondary of 50.xxx.xxx..59.  The SmarterMail protocol settings have SMTP out configured to use 50.xxx.xxx.59.

Using MX Toolbox, I can see there is a reverse dns PTR record for one of the domains (no errors returned to me when sending from this domain).  The other domains don't have one.  If I traceroute the mail server domain names, one is coming back from 50.xxx.xxx.59 and the other from 50.xxx.xxx.52.

What have I done wrong?