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I have a problem which I think can be resolved by adding a ptr record. Email isn't working for xx@domain_a.com. I looked up the smtp issues on mxtoolbox.com which returned the following: SMTP Reverse Banner Check Reverse DNS FAILED! This is a problem..

When I look at the DNS settings, the mail, mail(x) etc records point to an ip address and adomain.net. I believe the email is sent from an external server so the solution would be to add a PTR record, correct? Where do I add this? Do I add this on the domain to point to the email, or should it be added where the email is sent from to point to my domains ip?

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How about explaining what problem you're having so we can tailor our answers to the actual problem. –  joeqwerty Nov 13 '12 at 14:26
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If you have a large block of addresses, it's possible to have a reverse zone delegated. If you only have a handful of IPs that you "rent" from your ISP, then you'll have to have them create the record for you, as their servers likely hold the reverse lookup zone for all of the IPs that they rent out.

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hint: doing a recursive lookup on the PTR record should reveal the ns which has the SOA for the PTR record. However there are lots of SMTP servers without PTR records out there - publishing a restricting SPF may solve the problem more easily. –  symcbean Nov 13 '12 at 12:21
    
Really ? I'm glad I'm not responsible for any of them, then. Email requires FcrDNS, it's as simple as that. –  adaptr Nov 13 '12 at 12:45
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