Knowledgebase:
SMTP failure due to lack of PTR record
Posted by Francois Rufli, Last modified by Francois Rufli on 13 December 2014 06:00 PM
What is a reverse PTR record?
 
PTR record or more appropriately a reverse PTR record is a process of resolving an IP address to its associated hostname. 
This is the exact opposite of the process of resolving a hostname to an IP address. 
 
Example, when you ping a name mail.somedomain.com it will get resolved to the ip address using the DNS to something like 192.168.1.5. 
Reverse PTR record does the opposite; it looks up the hostname for the given IP address. 
In the example above the PTR record for IP address 192.168.1.5 will get resolved to mail.somedomain.com.
 
 
 
Is Reverse DNS Necessary?
 
Some junior DNS administrators configure forward DNS and forget to configure reverse DNS.
 
When they do this, some things work fine. Internet web browsing, for example, works great. However, not everything works.
 
Reverse DNS is required by some Internet protocols and by extensions to some other Internet protocols. Without reverse DNS, users will experience trouble with r-commands, IRC, SMTP servers, most enterprise management systems, and many network backup systems.
Troubleshooting problems that faulty or non-existent reverse DNS cause can take considerable time and effort.
It is much better to ensure that reverse DNS is configured correctly from the beginning.
 
 
How to check reverse PTR, reverse IP ?
 
How to check if your mail server DNS and ip is correctly configured to communicate with eSync.net mail servers ?
 
 
Examples below for the FQDN my.esync.net
 
 
1° get mail server fqdn for a domain
 nslookup -type=mx esync.net
 esync.net       MX preference = 10, mail exchanger = my.esync.net
 my.esync.net  internet address = 193.111.202.220
 
 
2° check name using ip 
 nslookup 193.111.202.220
 Name :    my.esync.net
 Address:  193.111.202.220
 
 
3° check reverse ip lookup
 nslookup -q=ptr 193.111.202.220
 220.202.111.193.in-addr.arpa    name = my.esync.net
 
 
More informations:
 
See RFC 1912, RFC 2505
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