Definition
Reverse DNS (rDNS), implemented through PTR (Pointer) records in DNS, maps an IP address to a hostname, performing the reverse of the more familiar forward DNS that maps hostnames to IPs. For email sending, PTR records serve as a validation mechanism: when your email server connects to a receiving mail server, the receiving server can perform a reverse lookup on your IP address to verify that the hostname claimed in the SMTP HELO/EHLO command matches a hostname associated with that IP.
Mailbox providers use PTR records as one of several signals in their reputation and authentication evaluation. A sending IP with a valid PTR record that matches the HELO hostname is treated more favourably than one without a PTR record or with a mismatched record. Gmail, Outlook, Yahoo, and other major providers all perform PTR checks. A missing or incorrect PTR record can result in increased deferral rates, lower inbox placement, or in some cases outright rejection. PTR configuration is considered a fundamental requirement for dedicated IP email sending.
PTR records are configured through your IP address provider or hosting provider, not through your DNS hosting provider. This is because PTR records for an IP address are managed by the entity that owns the IP address block. If you lease IPs from a hosting provider, they control the PTR record and you must request the change. If you own your IP space (via ARIN/RIPE/APNIC), you manage PTR records through your IP management system. PTR changes can take 24-72 hours to propagate fully due to the caching characteristics of reverse DNS.
Best Practices
Configure PTR records for every dedicated sending IP. Each IP address that sends email should have a PTR record that resolves to a hostname. The hostname should follow the format yourbrand-identifier.yourdomain.com (e.g., mailer1.example.com). Avoid generic hostnames like host-123-45-67-89.isp.com that are assigned by default by hosting providers.
Ensure PTR record matches the HELO/EHLO hostname. The hostname returned by the PTR lookup should match the hostname your MTA sends in the SMTP HELO/EHLO command. Exact match is best: if your HELO hostname is mailer1.example.com, the PTR for your IP should resolve to mailer1.example.com. Mismatched PTR and HELO is a common deliverability issue that mailbox providers flag.
Use a subdomain of your primary sending domain for PTR hostnames. Using mailer1.example.com rather than mailer1.thirdparty.com creates a consistent brand identity across all authentication signals. The PTR hostname should be within your authenticated sending domain (the domain in your SPF, DKIM, and DMARC records) to create alignment.
Verify PTR records regularly using dig or nslookup. Run dig -x <IP address> to check the PTR record for each of your sending IPs. Compare the returned hostname against your MTA's configured HELO hostname. Set up automated monitoring that alerts if any sending IP's PTR record changes unexpectedly or is removed.
Request PTR changes well in advance of launching new sending IPs. PTR record setup requires coordination with your IP provider and can take days. When ordering new IPs, request PTR configuration at the same time. Add PTR setup to your new IP deployment checklist. Sending from an IP without a properly configured PTR record will produce suboptimal deliverability from the start.
Related Glossary Terms
Bounce Classification
Bounce classification uses SMTP codes (550, 551, 552, 553, 554, 450, 451, 452) and enhanced status codes to categorise permanent and transient delivery failures.
DMARC Alignment
DMARC identifier alignment determines whether the domain in the From header matches the domains used in SPF and DKIM authentication. Strict or relaxed.
Email Active Subscriber
An active email subscriber has opened or clicked an email within a defined recency period, typically 30-90 days by industry. Active subscriber rate of 40-60% is typical for healthy email lists.
Email Bandwidth
Email sending bandwidth and throughput refer to the rate at which emails can be delivered, typically 100-500 emails/second per IP. Bandwidth planning ensures campaigns complete within desired delivery windows.
Email BIMI VMC
BIMI Verified Mark Certificate (VMC) certifies brand logo ownership for display in supporting email clients. VMCs cost £1,500-2,000+ per year per logo and require DMARC reject or quarantine policy plus SVG logo format.
Email Blacklist
An email blacklist (DNSBL) is a real-time database of IP addresses or domains known for sending spam or unwanted email.
Frequently Asked Questions
A PTR record maps an IP address to a hostname, enabling receiving mail servers to verify that your sending IP's identity matches the hostname you claim in the SMTP connection. It is part of the validation process that mailbox providers use to assess sender legitimacy and prevent spoofing.
Use the dig command: `dig -x
A missing or incorrect PTR record can result in: higher deferral rates as receiving servers treat the connection with suspicion, lower inbox placement rates, rejection by some receiving servers (particularly those with strict anti-spam configurations), and reduced sender reputation scores at major mailbox providers.
PTR records are managed by the entity that owns the IP address block. For most email senders, this is their hosting provider, ESP, or data centre. You must request PTR changes from your IP provider. If you own your IP space through a regional internet registry (ARIN, RIPE, APNIC), you can manage PTR records yourself.
PTR is not directly part of DMARC validation (which checks SPF and DKIM). However, PTR alignment supports overall sender authentication by providing consistency across all identity signals. Some mailbox providers use PTR/HELO alignment as an additional trust signal in their internal reputation scoring, which indirectly affects deliverability.