Proper reverse DNS configuration
Question asked by Rick Ryan - 11/5/2020 at 12:46 PM
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?

4 Replies

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Ron Raley Replied
Basically Reverse DNS is for the IP.  Reverse DNS must be established for any IP sending outgoing mail. It is an absolute must.

This isn't necessarily related to the "domains", however, when they send mail, the reverse ip record is viewed to verify your mail server name and ip match.

Rick Ryan Replied
OK.  Since Comcast "owns" the ip addresses being used, I assume they must setup the reverse DNS.  Can more than one name be returned for a single IP address?  Is it the case that the value(s) returned should reflect the name of the mail server (eg mail.mydomain.com and mail.myotherdomain.com) or just the the domain (eg mydomain.com and myotherdomain.com)?
Kyle Kerst Replied
Employee Post
Rick; I do believe Comcast would need to make the changes as they own the IP and likely manage its PTR records. It is also not possible to have more than one hostname returned for PTR records. In this case you would need separate IPs for each domain, or would need to select a primary hostname and use this for the PTR record and MX records. Hope that helps!
Kyle Kerst
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
Rick Ryan Replied
Thanks Kyle.  I cannot find where I can configure a specific IP address to bind to SMTP domain by domain such that mail sent from mydomain.com uses .52 and mail sent from myotherdomain.com originates from .59.  Is that possible with v15.7?

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