Are you seeing this error message when trying to send emails to Gmail?
550-5.7.25 The IP address sending this message does not have a PTR record setup, or the corresponding forward DNS entry does not match the sending IP. As a policy, Gmail does not accept messages from IPs with missing PTR records. For more information, go to https://support.google.com/a?p=sender-guidelines-ip To learn more about Gmail requirements for bulk senders, visit https://support.google.com/a?p=sender-guidelines
If so, Google's rejecting your mail with an alert that the DNS for your email sending infrastructure is misconfigured. Specifically, you're missing the hostname mapping for IP addresses that allows lookup of a server's hostname by querying the IP address. This is done by way of a PTR record in DNS, and Google can't find yours.
What is a PTR record?
A PTR (Pointer) record in the Domain Name System (DNS) facilitates the function of a reverse DNS lookup. It is essentially the opposite of the more common A record:
A Record (Forward DNS Lookup): Translates a domain name (like www.example.com) into its corresponding IP address (like 192.0.2.1). This is what happens when you type a website address into your browser.
PTR Record (Reverse DNS Lookup): Translates an IP address (like 192.0.2.1) back into its corresponding domain name (like mail.example.com).
Simply put, it's part of the mapping between server hostnames and IP addresses.
Functioning Forward-and-reverse DNS
It's long been a best practice to ensure that your email sending infrastructure transmit email messages only from servers where their IP addresses have properly resolving forward and reverse DNS, meaning that there are no "bare IPs addresses" lacking DNS, and that there is always the proper connection between looking up forward DNS (via an A record) and reverse DNS (via a PTR record).
Why? A mail server on an IP address without properly configured DNS is like a car without a license plate. Most people register their cars and display the proper license plate. Cars that lack that are more likely to be up to no good. The same is true for a mail server sending on an IP address with no PTR record. It can happen by accident -- a license plate can fall off -- but most good people don't intentionally drive around this way.
Thus, a broken or missing PTR record is a pretty significant spam sign.
Google now blocks based on PTR failures
This specific error code from Gmail (550-5.7.25) isn't entirely new; I've had people asking me about it for at least a few years now. But what is new is this: Google recently ramped up enforcement of their email sender requirements, meaning that it was somewhat rare for them to block because of this in the past, but no more; they're now quick to block, if your DNS is broken or lacking.
This is one of many recent examples of the largest mailbox providers (like Microsoft, Apple, Google and Yahoo) taking what were understood to be "best practices" and converting them into documented and mandated requirements that senders must comply with, if they want to see their email messages delivered to the inbox.
What to do about it
This came up on the Mailop list recently, where an email sender was sure that their email infrastructure had proper DNS in place and that the error must have been a false positive. Other list participants reviewed the DNS in question and found a complex delegation issue, suggesting that the rejections were not actually false positives.
If you're seeing email messages rejected with this error; it's time to look into the DNS settings for your email infrastructure. Either necessary bits of DNS are missing, or delegation's not working properly, or necessary servers aren't responding. It can be tricky to troubleshoot; intermittent occurrence could be a sign that only some, and not all, if your DNS servers are misconfigured or problematic.
This also highlights the importance of DNS monitoring, so that you, or your provider, can be sure that their infrastructure is fully configured properly and working as designed. A lot of people seem to treat DNS as "set it and forget it," rarely engaging in ongoing monitoring or periodic review. That's a risky way to run things nowadays.
Are you seeing this error message when trying to send emails to Gmail?
550-5.7.25 The IP address sending this message does not have a PTR record setup, or the corresponding forward DNS entry does not match the sending IP. As a policy, Gmail does not accept messages from IPs with missing PTR records. For more information, go to https://support.google.com/a?p=sender-guidelines-ip To learn more about Gmail requirements for bulk senders, visit https://support.google.com/a?p=sender-guidelines
If so, Google's rejecting your mail with an alert that the DNS for your email sending infrastructure is misconfigured. Specifically, you're missing the hostname mapping for IP addresses that allows lookup of a server's hostname by querying the IP address. This is done by way of a PTR record in DNS, and Google can't find yours.
What is a PTR record?
A PTR (Pointer) record in the Domain Name System (DNS) facilitates the function of a reverse DNS lookup. It is essentially the opposite of the more common A record:- A Record (Forward DNS Lookup): Translates a domain name (like www.example.com) into its corresponding IP address (like 192.0.2.1). This is what happens when you type a website address into your browser.
- PTR Record (Reverse DNS Lookup): Translates an IP address (like 192.0.2.1) back into its corresponding domain name (like mail.example.com).
Simply put, it's part of the mapping between server hostnames and IP addresses.Functioning Forward-and-reverse DNS
It's long been a best practice to ensure that your email sending infrastructure transmit email messages only from servers where their IP addresses have properly resolving forward and reverse DNS, meaning that there are no "bare IPs addresses" lacking DNS, and that there is always the proper connection between looking up forward DNS (via an A record) and reverse DNS (via a PTR record).Thus, a broken or missing PTR record is a pretty significant spam sign.
Google now blocks based on PTR failures
This specific error code from Gmail (550-5.7.25) isn't entirely new; I've had people asking me about it for at least a few years now. But what is new is this: Google recently ramped up enforcement of their email sender requirements, meaning that it was somewhat rare for them to block because of this in the past, but no more; they're now quick to block, if your DNS is broken or lacking.What to do about it
This came up on the Mailop list recently, where an email sender was sure that their email infrastructure had proper DNS in place and that the error must have been a false positive. Other list participants reviewed the DNS in question and found a complex delegation issue, suggesting that the rejections were not actually false positives.This also highlights the importance of DNS monitoring, so that you, or your provider, can be sure that their infrastructure is fully configured properly and working as designed. A lot of people seem to treat DNS as "set it and forget it," rarely engaging in ongoing monitoring or periodic review. That's a risky way to run things nowadays.
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