IPv6 reverse DNS for Comcast
Question asked by E. Keith Dodd - September 22, 2017 at 7:49 AM
Unanswered
Periodically, users of my mail server find mail to a Comcast address rejected: cause has to do with no reverse DNS record. Tests on MX sites, show all is OK. On Googling this, sounds like a common problem. Some say to "disable" IPv6 to solve it.
I have SM 16, v: 16.2.6439. My IP for mail server is 64.118.70.228 with name mail.wingserv.com. My IPv4 setting says 64.118.70.228. That for IPv6 says  'Use primary on NIC'. 
I'm not a mail expert, but am at a loss to determine the problem or a fix. Weird that only with Comcast and probably only 10% of time with Comcast. Other 90%, all goes through fine with Comcast.
Appreciate any insight and help.
Keith

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Merle Wait Replied
So.... usually... reverse DNS (where it is rejecting), has to do with the backwards DNS, pointing to your server, right?
In other words your server sets at   123.123.123.012.   Your server's host name is : mail.hostname.com   ... as an example,    Your provider (lets say Level 3),  would be required, to have put in a DNS reverse pointer, so that if anyone types  NSLookup 123.123.123.012, that it resolves to mail.hostname.com.  In order for that to happen, Level 3, in this case, has to establish that backwards PTR and name to resolve to.
 
When your server sends out email, then it has to be sure and send out over  123.123.123.012.   If it is sending through a firewall, then you would need to make sure that the firewall, is indicated 123.123.123.012
 
Depending on what is going on your users, and the settings on your mail server, it could be that their I/P address (which may or may not be in your email - you can set this at the domain level) .. could be be the issue.  Would be interesting to know if it is the same set of users every time.
 
As for IPV6, true ... I have disabled that.. but I am also ensuring that the above is true to.
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J Lee Replied
I have this problem also, how do you remove the ipv6 from the IP Addresses table in Smartermail?

J. Sebastian Lee Service2Client LLC 6333 E Mockingbird Ste 147 Dallas, TX 75214 - 877.251.3273

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J Lee Replied
I have ipv6 turned off on my NIC card and smartermail is still pulling and listing ipv6 address in the IP Address list.

J. Sebastian Lee Service2Client LLC 6333 E Mockingbird Ste 147 Dallas, TX 75214 - 877.251.3273

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E. Keith Dodd Replied
Doing a NSLookup 64.118.70.228 from both my home computer and one running my SM server all show mail.wingserv.com, which I presume is correct.
Any checks of mx services show the correct relationship between the IP and the correct nameserver.
Am I understanding you to be saying that perhaps if the Comcast user I have sometimes trouble sending to might get an incorrect response to the NSLookup?

Also, is there a downside to disabling IPV6 in the Outbound SMTP settings?
Thanks
Keith
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J Lee Replied
Moving thread to Problem post

J. Sebastian Lee Service2Client LLC 6333 E Mockingbird Ste 147 Dallas, TX 75214 - 877.251.3273

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