Do PTR records and HELOs impact email deliverability?

Key findings

• PTR records: A valid PTR record (reverse DNS) is crucial for email deliverability. It allows receiving mail servers to verify that the IP address sending the email matches the domain it claims to be from, adding credibility to the sender.
• HELO/EHLO command: The hostname presented in the HELO/EHLO command during the SMTP handshake should ideally align with the sending IP's PTR record. Inconsistent HELO hostnames can raise suspicion among receiving servers.
• Subdomain recommendation: Many experts recommend that the PTR record's domain be a subdomain of the sender's main domain (e.g., mta.yourdomain.com if your domain is yourdomain.com) to enhance trust and simplify spam filtering for receiving servers.
• ISP variations: While some large ISPs (like Google) may be more lenient, smaller or more strict ISPs (such as Hotmail or iCloud) often place greater emphasis on consistent PTR records and HELO hostnames.
• Shared IP pools: On shared IP pools, custom PTR and HELO settings (matching your brand's domain) are typically not possible, as the ESP manages these. This generally doesn't pose an issue if the ESP maintains a good reputation.

Key considerations

• Consistency is key: Ensure that your sending IP's PTR record resolves back to the hostname specified in your HELO/EHLO command (and vice-versa). This rDNS circle (forward and reverse DNS) should close properly.
• Impact on blocklists: A misconfigured PTR record can lead to your emails being rejected or your IP address being placed on a blocklist. Receiving mail servers often check PTR records to deter spam and phishing.
• Email authentication synergy: While PTR records and HELOs are important, they are part of a larger authentication ecosystem that includes SPF, DKIM, and DMARC. All these elements contribute to sender reputation and inbox placement. You can learn more about how DNS interacts with email in this guide from DNS Made Easy.
• Custom vs. ESP domains: If your ESP allows, setting the PTR for your sending IPs to a subdomain of your own brand can be beneficial for specific ISPs that are stricter about domain alignment. However, for Email Service Providers (ESPs) with good reputations, using their platform's domain for these settings often does not negatively impact deliverability.

Key opinions

• PTR importance: While some marketers note that large providers like Google seem less strict about PTR alignment with the sending domain, others emphasize its value for smaller ISPs like Hotmail and iCloud.
• HELO's role: The HELO hostname is recorded in email headers and can influence deliverability if it belongs to an ESP with a poor reputation, but this is usually a symptom of larger issues.
• Reducing data points: Some believe that using a branded PTR and HELO (subdomain of the sender's domain) can reduce the number of different domains a spam filter needs to evaluate, potentially improving deliverability.
• List-Unsubscribe: The domain used for the List-Unsubscribe mailto header is generally not considered a critical factor for email deliverability.

Key considerations

• Provider-specific nuances: What works for one ISP might not be as critical for another. Marketers note variations, such as Google being less finicky about PTR exact matches compared to Hotmail or iCloud.
• Overall sender reputation: A strong sender reputation (built on consistent sending practices, low complaints, and proper authentication like SPF and DKIM) often outweighs minor PTR/HELO misalignments.
• Shared IP limitations: On shared IPs, the ESP controls PTR and HELO. If deliverability issues arise, they are more likely due to overall pool reputation than these specific settings.
• Testing and monitoring: Regularly testing email deliverability across various ISPs and monitoring feedback loops can help marketers identify if PTR or HELO configurations are causing specific issues. You can use an email deliverability tester for this purpose. This article on DNS & Email Deliverability from Kickbox Blog provides further insights.

Key opinions

• rDNS closure: Experts universally agree that as long as the rDNS circle (forward and reverse DNS) closes correctly, the basic technical requirement is met.
• HELO reputation: The HELO hostname being from an ESP with an 'abysmal reputation' can negatively impact an email, though this usually indicates deeper underlying problems beyond just the HELO itself.
• Data point reduction: Changing PTRs and HELOs to align more closely with the sender's domain (if possible) can reduce the number of distinct data points an email presents, potentially making it appear more trustworthy to spam filters.
• Shared IP constraints: Experts acknowledge that branded PTR and HELO settings are typically not achievable on shared IP pools, and this is a common operational limitation.

Key considerations

• Foundational element: PTR records are a fundamental part of the email authentication process, ensuring that the IP address used for sending an email has a valid reverse DNS entry. This is a basic expectation for legitimate mail servers.
• Spoofing prevention: A properly configured PTR record enhances email deliverability by confirming authenticity, which helps to prevent spoofing attacks and build sender trust.
• Interaction with other DNS records: PTR records work in conjunction with other DNS records like A records and MX records to ensure the overall health and functionality of your email server and proper email routing. For more details, consider reading CloudNS's blog on reverse DNS.
• When HELO is an issue: If your HELO hostname is actively hurting your deliverability, experts suggest that it's likely indicative of broader domain reputation issues or poor sending practices that need addressing.

Key findings

• RFC 5321 (SMTP): This RFC defines the HELO/EHLO command, stating that the sending SMTP client should present its fully-qualified domain name (FQDN). This FQDN is typically expected to have a corresponding PTR record.
• Reverse DNS lookup: Many modern mail servers perform reverse DNS lookups (PTR queries) on incoming connections. If no PTR record exists or if it doesn't match the HELO/EHLO FQDN (or the forward DNS of the HELO domain), it can trigger a soft or hard rejection.
• Spam filtering signal: Lack of proper rDNS configuration is a common characteristic of spam and malicious email, thus legitimate senders are expected to have it correctly set up to avoid being categorized as suspicious.
• Trust and reputation: Proper PTR records and HELO configuration contribute to the overall trust and reputation score of a sending IP address and domain, influencing inbox placement.

Key considerations

• Server-side configuration: PTR records are configured at the IP address level, usually by your hosting provider or ISP (Internet Service Provider). You typically cannot set them yourself unless you own the IP address block.
• DNS consistency: It is best practice for the PTR record of the sending IP to resolve to a hostname that also has a forward A record pointing back to that same IP address. This bi-directional resolution (often called rDNS lookup) is a strong trust signal.
• HELO FQDN: The HELO or EHLO command should always present a fully qualified domain name (FQDN), not an IP address or a generic hostname, to adhere to best practices for SMTP communication.
• Interplay with other standards: While not explicitly part of SPF or DKIM alignment, the credibility established by correct PTR and HELO settings complements these authentication methods, building a stronger overall sender profile.