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2 Messages

Thu, May 7, 2015 4:00 AM

PTR Problem with static IP?



I have several static IP addresses from comcast. The one that I am having an issue with is (removed for account security).


Comcast has previously created a PTR record for this address that resolves to


I host the A record for and it resolves to  (removed for account security).


When I do IP lookups for that address using comcast or google servers, they resolve as I expect. Everything has been working fine for several years.


Roughly 2 months ago, "something" changed. When I send e-mail from that server, it no longer appears to be coming from  (removed for account security). It NOW appears to be coming from an IPv6 address.


This is the relevant line from the e-mail headers:

Received: from ([2601:5:c680:8600:20c:29ff:fe9b:16d7])

This is causing a problem as the comcast mail servers will not accept e-mail from servers that are not configured with correct PTR records and that IPv6 address does not resolve to 


I would like to understand what has changed and why it only affects my e-mail traffic as opposed to web and other traffic. What sort of network kung-fu do you have going on?


Ideally, "something" would change back to the previous behavior and mail would appear to come from  (removed for account security)again (along with my other  (removed for account security) IP addresses). Otherwise, I will need to know what options I have from comcast. (Presumably you can set the IPv6 address to resolve to as well but I will have to make additional changes on the mail server or dns server to support whatever is happening.)


Thank you for helping with this. I apologize if I am unclear. I can provide additional information if needed.


Craig Zody





2 Messages

6 years ago

Sorry for the confusing title. I spent 2 weeks thinking that this was an issue with the PTR record before realizing that something else very funky (to my way of thinking) was going on.


I still don't know how to describe what's really happening though since I don't understand myself.


Any pointers to help would be greatly appreciated.


Craig Zody

Trusted Forum Contributor


1.4K Messages

6 years ago

Hello CraigZody and welcome,


First suggestion I would provide is that you need to check your zone file because from the following there are some records that are missing:



CNAMENo CNAME record found.
PTRNo PTR record found.
2012051500 86400 3600 604800 10800



Second suggestion I would provide is for you to disable or turn off your Comcast Gateway (CG) LAN IPV6 functionality and only utilize your LAN IPV4. This can be done by logging into you CG, click on LAN then IPV6 tab and uncheck parameters to disable.


Hope this helps you out.



New Member


1 Message

6 years ago



I just ran into this same problem.  I am using Linux as the OS and Sendmail as the SMTP/Mailer.


The issue is that your server OS is running dual stack IP; IPv4 and IPv6.  Sometime within the past several months Comcast added IPv6 MX records for their mail servers.  I can't speak for other SMTP/Mailer software, but Sendmail when configured for IPv6 (which I believe is now a default setting) will default to the IPv6 MX record over the IPv4 MX record.  This is where the IPv6 address is coming from, it's the one dynamically assigned to your server.  At this time Comcast is way behind on assigning static blocks of IPv6 so that we can get PTRs and start using them.  You will also note in your log files that Gmail has been happly accepting delivery of your mail over the IPv6 address this entire time.


How to solve the issue is dependant on your SMTP/Mailer software.  In my scenario, you can configure Sendmail to deliver mail on a per domain basis to an IPv4 address. The catch is that this configuration is static, so if their mail server changes IPv4 addresses you will need to change it in the configuration.  I imagine that any Lunix/Unix based SMTP/Mailer will have similar configuration options.  Windows based ones not so sure of.  The other solution is to disable the IPv6 stack on the server. Disabling it network wide seems a bit heavy handed.


My Sendmail solution;

Sendmail - How To Deliver To IPv4 Address Per Domain



New problem solver


38 Messages

6 years ago

I'm running into the same thing, I've disales ipv6 in the is and on all send/receive connectors. Exchange 2013. Also my firewall is single stack.