Why is reverse DNS important for avoiding spam filters?
Michael Ko
Co-founder & CEO, Suped
Published 14 Jul 2025
Updated 18 Aug 2025
7 min read
When you send an email, many factors determine whether it lands in the recipient's inbox or gets caught in a spam filter. While common authentication methods like SPF, DKIM, and DMARC often take center stage, there's another crucial element that plays a silent, yet significant, role: reverse DNS (rDNS).
Reverse DNS is essentially the counterpart to the more commonly understood forward DNS lookup. While forward DNS translates domain names into IP addresses, rDNS performs the opposite function, mapping an IP address back to a hostname. This mapping is vital for verifying the legitimacy of sending servers.
Many email providers and spam filters rely on reverse DNS to establish trust. Without a properly configured reverse DNS entry, your emails risk being flagged as suspicious, deferred, or even outright rejected. This can severely impact your email deliverability rate and sender reputation.
Understanding reverse DNS and PTR records
To grasp why reverse DNS is so important, it helps to understand what it is and how it works. When you type a website address like suped.com into your browser, the Domain Name System (DNS) translates that human-readable name into a numerical IP address that computers understand. This is a forward DNS lookup. Reverse DNS does the opposite, taking an IP address and resolving it back to a domain name or hostname.
The specific record type used for reverse DNS is called a Pointer Record, or PTR record. Unlike A records, which map a domain to an IP, PTR records map an IP to a domain. These records are managed by the entity that controls the IP address space, typically your internet service provider (ISP) or hosting provider.
Example of a PTR recordDNS
1.2.3.4.in-addr.arpa. IN PTR mail.yourdomain.com.
A key aspect of a proper reverse DNS setup is Forward Confirmed Reverse DNS (FCrDNS). This means that not only does the IP address resolve to a hostname via a PTR record, but a forward DNS lookup of that hostname also resolves back to the original IP address. This two-way verification creates a strong chain of trust, which is highly valued by receiving mail servers.
Type
Function
Record type
Forward DNS
Translates a domain name to an IP address.
A or AAAA.
Reverse DNS
Translates an IP address to a domain name (or hostname).
PTR.
How spam filters use reverse DNS
Email servers (specifically, Mail Transfer Agents or MTAs) commonly perform reverse DNS lookups on the sending IP address as part of their initial verification process. This is a crucial step in their anti-spam efforts. If an incoming email originates from an IP address without a valid PTR record, or if the rDNS lookup fails, it raises a red flag.
Many spam filters and blocklists (or blacklists) use this information to assess the legitimacy of the sender. Spammers often use compromised machines or dynamic IP addresses without proper PTR records, making rDNS an effective tool for identifying and filtering out malicious or unsolicited mail. You can learn more about how email blocklists actually work in our detailed guide.
The verification process
When your mail server attempts to connect with a recipient's server, the recipient's server might perform a reverse DNS lookup on your sending IP address. It then compares the resulting hostname with the hostname provided in your server's HELO/EHLO command. A mismatch or the absence of an rDNS record can trigger spam filters and lead to delivery issues. This is why rDNS matching the SMTP banner is so important.
According to Akamai, rDNS is primarily used for security purposes to identify and trace the domain or ISP associated with an IP address, which helps in identifying spam sources. It's a foundational check that many receiving servers perform before even looking at other authentication methods.
The impact of a missing or misconfigured rDNS
Failing to properly configure reverse DNS can have several negative consequences for your email deliverability. Mail servers are designed to be cautious, and an absence of a PTR record or a mismatch can immediately cast doubt on your sending legitimacy.
Firstly, your emails are much more likely to be sent to the spam folder. Many spam filters are configured to give emails from IPs without valid rDNS a higher spam score. This increased score can tip the balance, leading to your legitimate messages being incorrectly classified as junk. This is one of the common reasons why emails go to spam.
Secondly, emails might be deferred or outright rejected. Some highly strict email providers or corporate mail servers may have policies to immediately reject any connection from an IP address lacking a proper rDNS entry. This means your emails won't even have a chance to reach the recipient's spam folder, let alone their inbox.
Impact of a missing rDNS
Increased spam score: Emails are more likely to be marked as spam.
Reduced trust: ISPs (like GMX) often reject emails from generic rDNS to combat botnets and bulk unsolicited mail. This type of shared rDNS can be an issue.
Finally, a lack of proper rDNS can lead to your sending IP being added to an email blocklist or blacklist. If an IP frequently sends mail without appropriate rDNS, it's quickly flagged as potentially spammy by various blacklists. Once on a blocklist, your emails will face widespread rejection, regardless of other authentication settings. This is a common reason for Spamhaus blocklistings.
Best practices for rDNS configuration
Ensuring your reverse DNS is correctly configured is a fundamental step towards robust email deliverability. Here are the key best practices:
Dedicated IP addresses: If you send a high volume of emails, using a dedicated IP address gives you full control over your rDNS. For shared IP environments, your ESP (Email Service Provider) should manage rDNS appropriately, but it's essential to understand how shared rDNS is configured.
Matching hostnames: Your PTR record should resolve to a hostname that accurately reflects your sending domain. For example, if your sending domain is yourdomain.com, your PTR record might be mail.yourdomain.com.
FCrDNS verification: Ensure that the hostname specified in your PTR record also resolves back to the originating IP address via a forward (A) record. This FCrDNS setup is a strong indicator of legitimacy.
Avoid generic rDNS: As seen with providers like Vsys Host, using generic hostnames assigned by your hosting provider can raise suspicion. Always request a custom rDNS entry that clearly identifies your domain. This is considered a best practice for reverse DNS.
Regularly check your rDNS configuration to ensure it is correct and hasn't been inadvertently changed. Misconfigured or missing rDNS can lead to significant deliverability problems. You can often request changes to your PTR record directly from your hosting provider or ISP.
Views from the trenches
Best practices
Always set a custom PTR record that clearly reflects your sending domain or brand.
Ensure your PTR record is configured for FCrDNS where possible, providing two-way verification.
Regularly monitor your rDNS to prevent misconfigurations or accidental changes by your provider.
If using a new IP, warm it up correctly and verify its rDNS before sending high volumes.
Common pitfalls
Relying on generic rDNS assigned by a hosting provider, which can be flagged by spam filters.
Not configuring rDNS at all, leading to immediate rejections by strict mail servers.
Mismatches between the rDNS hostname and the HELO/EHLO hostname used by your mail server.
Forgetting to update rDNS when changing IP addresses or mail server configurations.
Expert tips
A proper, non-generic rDNS improves sender trust, especially for services like Cloudmark.
Even if other authentications are strong, rDNS is a foundational check that can cause rejections.
Some providers may tighten rDNS requirements over time, even for existing configurations.
Dedicated IPs offer better rDNS control than shared environments, reducing deliverability risks.
Marketer view
Marketer from Email Geeks says: Some providers might classify reverse DNS with patterns like 'smht-117-16.dattaweb.com' as a static IP and then reject the mail, especially if it doesn't clearly identify the sender.
2020-07-24 - Email Geeks
Expert view
Expert from Email Geeks says: Generic reverse DNS is not a suitable configuration for sending mail.
2020-07-24 - Email Geeks
Solidifying your sender reputation
In the complex world of email deliverability, every piece of the puzzle matters. While SPF, DKIM, and DMARC handle domain authentication, reverse DNS serves as a foundational layer, verifying the identity of the sending IP address. It's a fundamental trust signal that many major email providers and spam filters look for.
By ensuring your reverse DNS is correctly set up, resolves properly, and ideally supports FCrDNS, you significantly enhance your sender reputation. This proactive step helps prevent your legitimate emails from being mistaken for spam, ensuring they reach their intended recipients. Ignoring rDNS can lead to unnecessary delivery challenges and undermine your overall email program.
ISPs (like GMX) often reject emails from generic rDNS to combat botnets and bulk unsolicited mail. This type of shared rDNS can be an issue.
