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What is reverse DNS (rDNS) and FCrDNS, and how do they impact email sending?

11 Marketer opinions5 Expert opinions4 Technical articles6 Resources

Summary

Reverse DNS (rDNS) translates IP addresses to domain names, verifying a mail server's authorization to send emails. FCrDNS (Forward-confirmed reverse DNS) ensures both forward and reverse lookups match, adding an extra layer of trust. Proper rDNS configuration, involving PTR records, is essential for email deliverability and authentication, preventing spoofing and improving sender reputation. Many receiving servers perform rDNS lookups and may reject emails without valid or matching records. While providers often manage FCrDNS, using static IPs with correctly configured rDNS is recommended, especially since dynamic IPs often lack proper setup, leading to deliverability issues. Generic rDNS records, sending from machines not intended as mail servers, or mismatches between the sending domain and rDNS can result in spam flagging. If direct rDNS configuration isn't possible, using an SMTP relay is advised. Google checks PTR records to ensure they align with the EHLO domain during connection, improving deliverability rates.

Key findings

  • rDNS Definition: rDNS translates IP addresses to domain names for sender verification.
  • FCrDNS Validation: FCrDNS confirms both forward and reverse DNS records match, enhancing trust.
  • Deliverability Boost: Proper rDNS configuration improves email deliverability and sender reputation.
  • Authentication Role: rDNS helps authenticate sending servers and prevent email spoofing.
  • Dynamic IP Problems: Dynamic IPs often lack correct rDNS, causing deliverability issues.
  • PTR Record Importance: PTR records in the in-addr.arpa domain link IP addresses to domain names.
  • Mismatched rDNS Impact: Mismatched or generic rDNS entries can lead to emails being flagged as spam.

Key considerations

  • Static IP Preference: Use a static IP with correctly configured rDNS for reliable email sending.
  • Provider Responsibility: Verify your provider handles FCrDNS, especially with services like Gmail.
  • SMTP Relay Alternative: Consider an SMTP relay if direct rDNS configuration isn't feasible.
  • Dedicated Setup: Opt for a dedicated IP address and hostname with proper rDNS to maximize deliverability.
  • Monitoring Necessity: Regularly monitor rDNS records to ensure they are correctly configured and resolve to the expected domain name.

What email marketers say
11Marketer opinions

Reverse DNS (rDNS) is the inverse of forward DNS, translating IP addresses to domain names. FCrDNS (Forward-confirmed reverse DNS) ensures that both forward and reverse DNS records match. These records are crucial for email deliverability and authentication. rDNS confirms that the sending server is authorized to send emails for a particular domain, preventing spoofing and improving sender reputation. Many receiving servers perform rDNS lookups and may reject or flag emails without valid or matching records. Using static IPs with proper rDNS setups is recommended for reliable email sending, while dynamic IPs often lead to deliverability issues. If rDNS cannot be configured, using an SMTP relay is advised. FCrDNS is considered an important trust signal and a discrepancy between the sending domain and RDNS may be marked as spam.

Key opinions

  • rDNS Definition: rDNS translates IP addresses to domain names, verifying the sender's authorization.
  • FCrDNS Definition: FCrDNS means your hostname has both an A record resolving to your IP and a PTR record resolving back to your hostname.
  • Deliverability Impact: Valid rDNS records improve email deliverability and prevent emails from being flagged as spam.
  • Authentication Role: rDNS helps email servers authenticate the sender and prevent spoofing.
  • Dynamic IP Issues: Dynamic IPs often lack proper rDNS, leading to deliverability problems.
  • Mismatch impact: A discrepancy between the sending domain and RDNS may be marked as spam.

Key considerations

  • Provider Responsibility: Email and CRM providers often handle FCrDNS setup, but it's essential to verify.
  • Static IP Recommendation: Using a static IP with correctly configured rDNS is recommended for reliable email sending.
  • SMTP Relay Option: If rDNS cannot be configured, consider using an SMTP relay service.
  • Monitoring rDNS: Regularly monitor rDNS records to ensure they are correctly configured and match sending domains.
Marketer view

Email marketer from EasyDMARC shares that FCrDNS (Forward-confirmed reverse DNS) means that your hostname has both an A record that resolves to your IP address and a PTR record that resolves back to your hostname. It's an important trust signal for email servers.

August 2024 - EasyDMARC
Marketer view

Email marketer from MXToolbox responds that when an email is sent, the recipient server performs an rDNS lookup on the sending server's IP address. If the rDNS record is present and matches the sending domain, it increases the likelihood that the email will be accepted.

July 2024 - MXToolbox
Marketer view

Marketer from Email Geeks explains that when the MTA connects to Google, it will say ehlo mta1.yourdomain.com, and Google will check the IP, retrieve the PTR record, and expect it to match.

December 2023 - Email Geeks
Marketer view

Email marketer from Email Marketing Forum explains that mail servers that have a reverse dns entry that points to a domain name that does not match the sending domains will typically be marked as spam by mail providers.

September 2022 - Email Marketing Forum
Marketer view

Email marketer from StackExchange highlights that email servers using dynamic IPs often have generic or non-existent rDNS records, which can cause email delivery issues. It's generally recommended to use a static IP with a proper rDNS setup for reliable email sending.

May 2021 - StackExchange
Marketer view

Email marketer from SparkPost Blog shares that rDNS helps improve email deliverability by confirming that the sending server is authorized to send emails on behalf of the domain in the 'From' address. A mismatch or absence of rDNS can flag emails as suspicious.

March 2023 - SparkPost Blog
Marketer view

Email marketer from WebHostingTalk Forums responds that if you are unable to setup a reverse dns record due to restrictions from your web hosting company, you should send mail via a SMTP relay as you cannot guarantee deliverability without one.

December 2021 - WebHostingTalk Forums
Marketer view

Marketer from Email Geeks shares that a PTR record is the reverse of an A record. If mta1.yourdomain.com points at 10.1.1.2, there would be a PTR record for 10.1.1.2 that points to mta1.yourdomain.com.

January 2025 - Email Geeks
Marketer view

Email marketer from Mailjet Blog responds that rDNS is crucial for email authentication. When an email server receives a message, it performs an rDNS lookup to verify if the IP address matches the domain sending the email. This helps to prevent spoofing and phishing attacks.

October 2021 - Mailjet Blog
Marketer view

Marketer from Email Geeks explains that your provider (email, CRM) should take care of the FCrDNS requirement, as the requirement is that sending MTAs (email servers) have FCrDNS.

February 2025 - Email Geeks
Marketer view

Email marketer from Reddit shares that having a proper rDNS setup significantly improves your email sending reputation. Many receiving mail servers will reject or flag emails from servers without a valid rDNS record.

October 2024 - Reddit

What the experts say
5Expert opinions

rDNS and FCrDNS are crucial for verifying the legitimacy of email servers. rDNS translates IP addresses to domain names, while FCrDNS ensures the forward and reverse lookups match. Generic rDNS records or their absence signal a non-dedicated or misconfigured server, potentially leading to spam flagging. Setting up proper rDNS for a dedicated IP address and hostname is recommended to ensure deliverability, as it confirms the server's authorization to send emails. Mail sent from machines not intended to be mail servers, such as those with consumer IP pools, are likely to encounter issues, and the goal of rDNS checks is to avoid the generic.

Key opinions

  • Generic rDNS Issues: Generic rDNS records or missing records indicate non-dedicated or misconfigured servers, increasing the risk of spam flagging.
  • FCrDNS Verification: FCrDNS verifies that the forward and reverse DNS records match, confirming the server's authorization.
  • Mail Server Identification: rDNS and FCrDNS play a vital role in identifying legitimate mail servers and preventing email spoofing.
  • RDNS impact: Servers without a RDNS record, or ones that contain a generic hostname, will likely be flagged as spam.

Key considerations

  • Dedicated IP Setup: Set up a dedicated IP address and hostname with proper rDNS to improve email deliverability.
  • Avoid Consumer IPs: Avoid sending email from machines with consumer IP pools or generic servers to minimize deliverability issues.
  • Monitor rDNS: Regularly monitor rDNS records to ensure they are correctly configured and resolve to the expected domain name.
Expert view

Expert from Word to the Wise explains that FCrDNS (full-circle reverse DNS) is the combination of forward DNS (a domain name resolves to an IP address) and reverse DNS (an IP address resolves back to the same domain name). It verifies that the server sending email is authorized to do so, improving deliverability.

September 2021 - Word to the Wise
Expert view

Expert from Email Geeks says that if you want to keep using the machine/IP address then you probably want to get it a nailed down IP address and hostname, and set up reverse DNS for it.

August 2021 - Email Geeks
Expert view

Expert from Email Geeks notes that the rDNS issue Dave is experiencing with Gmail is due to sending mail from a machine not intended to be a mail server. Generic reverse DNS indicates consumer IP pools or misconfigured generic servers.

February 2022 - Email Geeks
Expert view

Expert from Spamresource.com says that a mail server that does not have a RDNS record or the record contains a generic hostname is a good indication the server is dynamic or a zombie and as such email from those servers will likely be flagged as spam.

August 2022 - Spamresource.com
Expert view

Expert from Email Geeks clarifies that Google doesn't care if the reverse DNS is in your domain, it just wants it to not be loudly saying “I’m not a real mailserver, I’m a generic, unconfigured machine”.

February 2022 - Email Geeks

What the documentation says
4Technical articles

Reverse DNS (rDNS) resolves IP addresses to domain names, serving as the opposite of forward DNS. It's used to verify that an email server is authorized to send mail from a particular domain. rDNS configuration involves creating PTR records in the reverse DNS zone, which map IP addresses to domain names and are essential for email server identification. PTR records reside in the in-addr.arpa domain and are managed by the IP address block owner. For services like Amazon SES, configuring rDNS often requires a request to the service provider to set up the PTR record for dedicated IP addresses.

Key findings

  • rDNS Function: rDNS resolves IP addresses to domain names, verifying the sender's authorization.
  • PTR Records: rDNS is configured using PTR (pointer) records that map IP addresses to domain names.
  • in-addr.arpa Domain: PTR records reside in the in-addr.arpa domain, managed by the IP address block owner.
  • AWS SES Configuration: Configuring rDNS for Amazon SES typically requires a request to Amazon for PTR record setup.

Key considerations

  • IP Address Control: Setting up rDNS requires access to the IP's DNS settings or cooperation from the IP address block owner.
  • Service Provider Assistance: For some services, like AWS SES, the service provider must configure rDNS on your behalf.
  • Correct Configuration: Ensure that PTR records are correctly configured to map IP addresses to the appropriate domain names for proper email server identification.
Technical article

Documentation from AWS describes that to set up rDNS for Amazon SES, you must request Amazon to configure the PTR record for your dedicated IP addresses. This process verifies your control over the sending domain and improves deliverability.

November 2022 - AWS Documentation
Technical article

Documentation from Microsoft Learn explains that Reverse DNS (rDNS) performs the opposite function of forward DNS. It resolves an IP address to a domain/hostname. It's used to verify the server associated with an IP address is authorized to send emails from the domain.

June 2022 - Microsoft Learn
Technical article

Documentation from RFC 1912 details that rDNS configuration involves creating PTR (pointer) records in the reverse DNS zone. These records map IP addresses to domain names and are essential for proper email server identification.

June 2021 - RFC 1912
Technical article

Documentation from DigitalOcean Community explains that rDNS records, specifically PTR records, reside in the in-addr.arpa domain and are managed by the owner of the IP address block. Setting up rDNS requires access to the IP's DNS settings.

August 2023 - DigitalOcean Community

Related resources
6Resources

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