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If I perform an MX query on my domain (domain.com) I receive this answer:

Pref   Hostname         IP Address
5      mail.domain.com  12.34.20.145

If I perform a reverse DNS (PTR) query on 12.34.20.145 I receive:

Type    IP Address      Domain Name
PTR     12.34.20.145    firewall.domain.com

MX and PTR do not match each other, but our mail are not marked as spam. Is it working as expected?

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    Where does firewall.domain.com point to? – tim Apr 20 '16 at 18:54
  • You will get mixed results. It is up to the receiver to decide how to handle your emails. They could reject them, flag them in a header as spam, change the subject to spam, or do nothing. RFC1912 requires FCrDNS. Not everyone follows it and those that do differ on what to do with it. – Aaron Apr 20 '16 at 18:54
  • @Aaron RFC 1912 is Informational and defines neither an Internet Standard or a BCP. That's like saying that not everyone follows the advice of a magazine article. – Andrew B Apr 20 '16 at 19:25
  • Firewall.domain.com points to 12.34.20.145 too, it is a firewall that, by port forwarding, redirects port 25 to the internal mail server. – J.B. Apr 20 '16 at 22:44
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    Since the MX record designates which server receives email for your domain, not which server sends email for your domain, I'd say it's working perfectly. – joeqwerty Apr 20 '16 at 23:12
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You don't care what the MX record is compared to the rDNS record. You do care about the outgoing IP from your mailserver compared to the rDNS record for your domain. They may or may not be the same.

So make sure that whatever address the email server is actually sending from has a corresponding reverse pointer that resolves to your email domain name.

Most spam appliances/cloud services don't care, but some do, and one of them is Craigslist. Here's what a bounce back from CL looks like when you don't have a proper rDNS entry:

enter image description here

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  • So rDNS check are all about the server who is sending mail, and are not linked to MX record. If the server who is sending the email (not necessarily the one specified in the MX record) has a correct IP Address <-> rDNS record correspondance, the email message will be accepted (limited to rDNS spam checks)...Have I understood it correctly? – J.B. Apr 21 '16 at 13:12
  • Yes, you're right. However, this is just a small piece of the spam puzzle. And not really a terribly important one compared to some others. If the IP address is used for anything besides sending out emails, get yourself on mxtoolbox's blacklist check so you'll receive an email if/when your IP gets blacklisted due to a mailbot etc. – Tedwin Apr 21 '16 at 14:37

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