Reverse DNS (rDNS) is a crucial component of email deliverability, acting as the inverse of standard DNS. While forward DNS (A records) translates a domain name into an IP address, rDNS (specifically, PTR records) translates an IP address back into a domain name. FCrDNS, or Forward-Confirmed Reverse DNS, takes this a step further by requiring that the rDNS lookup (IP to domain) matches the forward DNS lookup (domain to IP). This dual verification process is widely used by receiving mail servers to authenticate sending servers, significantly impacting whether your emails land in the inbox or are flagged as spam.
Key findings
Fundamental authentication: rDNS and especially FCrDNS are essential for email authentication, providing a layer of trust that verifies the sending server's identity. Many receiving servers will reject mail if these checks fail.
Provider responsibility: For most users, particularly those sending through an Email Service Provider (ESP) or a cloud-based mail host like Gmail, the ESP is responsible for configuring and maintaining proper rDNS records. You can learn more about this in our article on rDNS best practices when sending via an ESP.
Impact on deliverability: A mismatched or missing rDNS record can significantly harm email deliverability, increasing the likelihood of emails being flagged as spam or outright rejected by recipient servers.
PTR records defined: A PTR record is essentially the reverse of an A record, mapping an IP address back to its associated domain name. We dive deeper into PTR records and their role in deliverability in a dedicated resource.
Key considerations
Self-hosting implications: If you operate your own Mail Transfer Agent (MTA) or use dedicated IP addresses, configuring and maintaining correct rDNS becomes your direct responsibility.
Generic rDNS: Generic reverse DNS entries, often associated with consumer IP pools or misconfigured servers, are a significant red flag for email receivers and can severely impact inbox placement.
Monitoring reports: Regularly check deliverability reports and diagnostic tools for any rDNS-related errors or warnings. These indicate potential issues that could be affecting your email campaigns.
Dedicated IP vs. shared IP: With shared IP addresses (common with many ESPs), rDNS is managed by the provider. If you have a dedicated IP, you will typically need to work with your hosting provider to set up the PTR record.
What email marketers say
Email marketers often find the technical aspects of rDNS and FCrDNS daunting, usually expecting their Email Service Providers (ESPs) to handle these configurations. However, issues can arise, especially when using specific email setups or monitoring tools that flag potential rDNS problems. Marketers are primarily concerned with the direct impact on their email campaigns and sender reputation, seeking clarity on who is responsible for these technical configurations.
Key opinions
Reliance on ESPs: Many marketers assume their email or CRM provider (like Hubspot or Amazon SES) automatically handles rDNS, and it shouldn't be their direct concern.
Confusion with reports: Some marketers receive monitoring reports indicating rDNS issues, even when using well-known services like Gmail for corporate email, leading to confusion and concern about their domain's reputation. This is sometimes connected to shared rDNS configurations.
Impact on reputation: A primary worry is that incorrect or missing rDNS (especially when sending regular corporate emails) could negatively impact their domain reputation and general email deliverability.
Key considerations
Verify ESP handling: Always confirm with your ESP how they manage rDNS for your sending IPs, especially if you use dedicated IPs.
Monitor reports carefully: If monitoring tools report rDNS issues, investigate whether these relate to your primary sending infrastructure or other sources. Reviewing email headers can clarify which IP is generating the alert.
Understand ownership: If you're not using a dedicated IP, your rDNS will likely point to your ESP's infrastructure, not your specific domain, which is normal and expected by most receiving mail servers.
Marketer view
Email marketer from Email Geeks asked about the Reverse DNS (PTR) record requirement, stating they hadn't fully understood it yet and were seeking clarification.
22 Jan 2024 - Email Geeks
Marketer view
Email marketer from Campaign Cleaner highlights that a server's reverse DNS must match its forward DNS, otherwise mismatches can lead to emails being flagged as suspicious or rejected.
25 Feb 2023 - Campaign Cleaner
What the experts say
Experts consistently emphasize that Forward-Confirmed Reverse DNS (FCrDNS) is a non-negotiable requirement for legitimate mail servers. They highlight that the absence or misconfiguration of rDNS, particularly generic rDNS entries, serves as a strong indicator of a non-standard or potentially malicious sending source. For most businesses leveraging ESPs, rDNS management falls squarely on the provider, but self-hosters or those with dedicated IPs must ensure proper setup to maintain deliverability.
Key opinions
MTA requirement: Sending Mail Transfer Agents (MTAs) are expected to have FCrDNS configured. If you're operating an MTA without knowing about FCrDNS, it suggests a significant gap in setup knowledge.
FCrDNS mechanics: When an MTA connects to a receiving server (e.g., Google), it announces its hostname via HELO. The receiving server then performs a PTR lookup on the connecting IP and expects it to match the announced hostname for FCrDNS validation. Our article on PTR records and HELOs explains this in more detail.
Generic rDNS is a red flag: If an IP's rDNS points to a generic hostname (e.g., a dynamic IP from a consumer pool or an unconfigured cloud node), it's a strong indicator that the machine is not intended to be a legitimate mail server, leading to deliverability issues. This is why missing or different rDNS impacts deliverability.
Key considerations
Provider vs. self-managed: Unless you bring your own IPs to a service or are running your own mail server, rDNS should typically be handled by your service provider (ESP or cloud host).
Dedicated IP setup: If you have a dedicated IP address for sending, ensure it has a proper, unique rDNS entry that matches your sending domain's forward DNS.
Google Workspace considerations: For standard Google Workspace (Gmail) users, rDNS configuration is handled by Google and is not expected to be managed by the end-user. If issues arise, it might indicate mail is being sent via an unexpected route or from a misconfigured local MTA rather than through Gmail's infrastructure. Google's new sender requirements reinforce the importance of these checks.
Expert view
Expert from Email Geeks states that your email or CRM provider should handle reverse DNS (rDNS), as the requirement is that sending MTAs (email servers) have Forward-Confirmed Reverse DNS (FCrDNS).
22 Jan 2024 - Email Geeks
Expert view
Expert from SpamResource explains that proper rDNS configuration is one of the foundational elements of good email deliverability, and its absence can cause legitimate emails to be treated as spam.
10 Mar 2024 - SpamResource
What the documentation says
Official documentation and industry standards highlight reverse DNS and FCrDNS as fundamental aspects of email server configuration for security, logging, and anti-spam measures. RFCs and prominent mail service providers (ISPs) often implicitly or explicitly require these checks. The core principle is to verify the identity of the sending server, ensuring that the IP address corresponds to a legitimate hostname. Failure to meet these expectations can lead to emails being rejected or classified as spam, impacting deliverability significantly.
Key findings
Anti-spam filtering: Reverse DNS is considered a primary level of anti-spam filtering, crucial for the proper delivery of emails, as noted by sources like Badsender.
Sender legitimacy verification: Email servers perform rDNS lookups to verify the sending server's domain name against its IP address, which helps reduce spam by ensuring the sender is legitimate, as explained by NetQuery.
Security and logging: Reverse DNS enhances network security, improves logging accuracy, and supports email authentication, often used to verify server identities before email transmission, according to JumpCloud.
Impact on deliverability: Reverse DNS significantly impacts email deliverability because many mail servers use rDNS to verify the sender's legitimacy upon email receipt. ClouDNS Blog states that a mismatch can raise flags, leading to emails being flagged as suspicious or rejected.
Key considerations
Conformity to standards: Adhering to FCrDNS best practices aligns your sending infrastructure with widely accepted email security and deliverability standards.
Provider coordination: For those using third-party IP addresses (e.g., from a cloud provider like AWS), coordinating with the third party is necessary to set up rDNS for your SMTP servers, as detailed by Amazon Web Services, Inc. documentation.
Mitigating rejections: Proper FCrDNS helps prevent emails from being blocked or sent to spam folders by recipient servers that strictly enforce these checks.
Fundamental DNS knowledge: Understanding how DNS records (A, PTR, MX) interact is crucial for troubleshooting and ensuring robust email infrastructure.
Technical article
Documentation from JumpCloud states that reverse DNS improves network security, logging accuracy, and email authentication, and is frequently employed to verify server identities prior to email transmission.
15 Sep 2023 - JumpCloud
Technical article
ClouDNS Blog documentation confirms that Reverse DNS significantly impacts email deliverability because many mail servers use rDNS to verify the sender's legitimacy upon receiving an email.