How does missing or different reverse DNS (rDNS) impact email deliverability, and when is a dedicated IP address needed?

Key findings

• rDNS importance: Missing rDNS can significantly impact deliverability, as mail servers (especially enterprise filters) often check for it as a basic spam prevention measure. It's a crucial component for establishing sender legitimacy.
• Round trip DNS: Mailbox providers, including major ones like Google and Yahoo, expect proper round trip DNS, meaning the IP address resolves to a domain name via rDNS, and that domain name then resolves back to the same IP address via a forward DNS lookup.
• Differing rDNS across ranges: Variations in rDNS across an ESP's IP range are not inherently problematic, provided the specific sending IP has a valid rDNS entry. Mail servers primarily evaluate the IP from which the email originates.
• Dedicated IP rationale: The decision to use a dedicated IP is less driven by rDNS configuration and more by the need for independent reputation management and higher sending volumes, often exceeding what a shared IP can reliably handle.

Key considerations

• Verify active sending IPs: When assessing rDNS issues on shared IPs, focus on the IPs actually used for sending emails. Inactive or reserved IPs without rDNS are unlikely to impact your deliverability.
• Impact of missing rDNS: A missing or incorrect rDNS record for an active sending IP is a significant deliverability hurdle. It prevents receiving servers from verifying the sender, often leading to rejections or spam folder placement. You may need to configure your reverse DNS correctly.
• Dedicated IP volume: For moderate volumes, such as 40,000 emails per week, a shared IP environment with good sending practices is often sufficient and preferable to a dedicated IP. Lower volumes on dedicated IPs can struggle to build sufficient reputation.
• Reputation is key: Ultimately, a sender's reputation, built through consistent good practices (low bounces, complaints, high engagement), outweighs minor rDNS anomalies on shared IPs, as long as the primary sending IP's rDNS is correctly configured.

Key opinions

• Confusion between return path and rDNS: There can be confusion between the email's return path, which is set in the email header, and reverse DNS, which is an IP-level record.
• Shared IP rDNS anomalies: When checking shared IP ranges, it's common to find differing rDNS entries or even missing ones across various IPs. This is a source of concern for marketers aiming for optimal deliverability.
• Volume for dedicated IPs: Marketers often weigh whether their sending volume is sufficient to warrant a dedicated IP, fearing that too low a volume might not build enough reputation to justify the cost and effort.
• Assumptions about IP usage: It's easy to assume that all IPs within an ESP's advertised range are actively used for sending, leading to unnecessary worry about rDNS configurations on unused IPs.

Key considerations

• Focus on actual sending IPs: Marketers should verify the rDNS of the specific IP addresses from which their emails are actually being sent, rather than the entire range specified in SPF records.
• Shared IP nuances: Even with shared IPs, it is important that the sending IP's rDNS is correctly configured. A common strategy involves using shared rDNS with a common hostname for the ESP, which is generally acceptable to ISPs.
• Dedicated IP thresholds: For volumes like 40,000 emails per week (around 160,000 per month), a dedicated IP may not always be necessary or even beneficial. Shared IPs can be effective if the ESP maintains a good reputation across the pool, as discussed in related discussions.

Key opinions

• Missing rDNS is critical: Missing rDNS may well impact delivery, especially as enterprise filters are pickier. Major providers like Yahoo and Google have made it a requirement for round trip DNS to verify sender legitimacy.
• Differing rDNS across ranges: Different rDNS across an IP range is not a problem in itself, though some specific patterns can be concerning. The focus should be on the actual sending IP.
• Volume for dedicated IPs: Sending volume matters less for dedicated IPs than it used to. Lower volumes on dedicated IPs may struggle to build reputation, making shared IPs a better option for moderate senders (e.g., 40,000 emails per week).
• rDNS and dedicated IPs: Concerns about rDNS should generally be excluded when evaluating the need for a dedicated IP address. As long as the IP points somewhere, strict rDNS domain matching is not always a make-or-break factor for deliverability anymore, especially for good senders.

Key considerations

• Evaluate based on actual sending: Mail servers only care about the rDNS details relevant to the actual sending IP, not the entire range specified via DNS, if that IP is within the range. Ensure your PTR records and HELO match.
• No upside for low volume dedicated IPs: Switching to a dedicated IP solely due to rDNS concerns discussed in this context (e.g., varying rDNS on shared IPs) offers no upside, especially for lower volumes like 40,000 emails per month. Such volumes are well-suited for shared IPs.
• Joining good sender pools: For moderate volumes, it can be more beneficial to join a pool of good senders on shared IPs, as this allows you to leverage the collective reputation without the burden of building a new dedicated IP reputation from scratch. Consider when to use shared vs dedicated IPs.

Key findings

• rDNS as anti-spam: Reverse DNS is widely considered the first level of anti-spam filtering. A correct rDNS configuration is crucial for good email delivery.
• Verification process: Reverse DNS allows mailbox providers to verify the sender during a reverse DNS lookup upon email receipt. This helps confirm the legitimacy of the sending server. This is a common and important email authentication method.
• Importance for deliverability: rDNS significantly impacts email deliverability because many mail servers use it to verify the sender's legitimacy. A missing or misconfigured rDNS can lead to emails being rejected or marked as spam.
• Dedicated IP control: When using a dedicated IP, you gain full control over your sender's reputation, as it is not influenced by other senders sharing the same IP. This also allows you to manage your own rDNS directly.

Key considerations

• FCrDNS check: A full-circle reverse DNS (FCrDNS) check confirms that an IP address resolves to a domain name, and that domain name then resolves back to the original IP address. This two-way verification is often expected.
• Troubleshooting missing records: If rDNS is missing, the IP address owner (often the ESP or hosting provider) must be contacted to create or update the reverse DNS record. This is a critical step in resolving deliverability issues caused by rDNS. This is also important for avoiding DNS-based blocklists.
• Shared vs. dedicated IP strategy: Documentation often outlines the benefits and drawbacks of shared and dedicated IP strategies, indicating that the choice depends on factors beyond just rDNS, such as sending volume, reputation goals, and the nature of email traffic (e.g., transactional vs. marketing).