How do I check if the sending IP address matches the IP address of the hostname specified in the Pointer (PTR) record?

Key findings

• FCrDNS is vital: A complete Forward Confirmed Reverse DNS setup, where the IP address resolves to a hostname, and that hostname resolves back to the same IP, is a strong signal of legitimacy for receiving mail servers. This process helps combat spam and spoofing.
• Deliverability impact: Proper PTR records and FCrDNS significantly enhance your sender reputation, leading to better inbox placement and reduced chances of being flagged as spam. Conversely, a mismatch or missing PTR record can lead to emails being rejected or sent to the spam folder.
• Not new, but newly emphasized: While PTR records and FCrDNS have always been part of good email practices, recent updates to sender guidelines from major providers have brought them into sharper focus, making them non-negotiable for high-volume senders.
• Verification methods: You can verify FCrDNS by performing a reverse DNS lookup on your sending IP address and then performing a forward DNS lookup on the resulting hostname to ensure they match.

Key considerations

• Shared vs. dedicated IPs: For shared IP addresses managed by an Email Service Provider (ESP), the PTR setup is typically handled by the ESP. For dedicated IPs, you or your network administrator are responsible for setting up and maintaining the A and PTR records.
• Ownership and control: The PTR record must be set by the owner of the IP address, usually your internet service provider (ISP) or hosting provider. You cannot set this record within your domain's DNS settings.
• Consistency with other records: While the PTR hostname doesn't need to match your sender or DKIM domain, it should reflect the legitimate sending identity of the server. For example, it might match the hostname in the SMTP HELO command.
• Google's emphasis: Google's email sender guidelines explicitly state the importance of PTR records, indicating that they check the sending IP address to ensure it matches the hostname specified in the PTR record.

Key opinions

• Shared IP reliance: Many marketers on shared IPs trust their ESPs to handle PTR record configurations, which is generally a safe assumption, though verification is still possible and recommended.
• Dedicated IP focus: The primary concern for marketers tends to be clients using dedicated IP addresses, as these often require manual DNS management which can be prone to errors or accidental deletions.
• Header insights: For quick checks, examining email headers (like in Gmail's 'Show Original' view) can reveal the sending IP, which then needs to be cross-referenced with its PTR record.
• Bulk verification need: Consultants managing multiple clients express a need for efficient, bulk PTR record checking tools to streamline verification processes.

Key considerations

• Dynamic shared IPs: Marketers on shared IPs should be aware that their sending IP can change or that they might be sending from multiple IPs, making consistent manual verification challenging.
• Manual DNS risks: There's a recognized risk of non-technical staff accidentally deleting or misconfiguring crucial DNS records (like PTR records) when managing dedicated IPs, highlighting the need for careful oversight.
• Beyond headers: While email headers show the sending IP, they don't directly confirm the PTR record's hostname. A separate lookup is required to ensure the full circle reverse DNS is complete.
• Proactive monitoring: Marketers should integrate PTR record checks into their routine deliverability monitoring, alongside blocklist monitoring and seedlist testing, to quickly identify and resolve any issues.

Key opinions

• FCrDNS is standard: Experts agree that FCrDNS is a long-standing best practice. Network engineers typically configure this automatically, making it a routine part of server setup for legitimate mail sending.
• Verification process: The core method involves looking up a hostname to get an IP, then looking up that IP to get a hostname. If these two hostnames match, the FCrDNS is confirmed.
• Header non-alignment: It's important to understand that the PTR record's hostname doesn't need to match anything in the email headers (like the 'From' address or DKIM domain).
• Tool availability: Various command-line and online tools can perform FCrDNS checks, with some even offering advanced scripting capabilities for automated verification.

Key considerations

• Smarthost configuration: Each smarthost used for sending email should have its own dedicated hostname with corresponding A and PTR records correctly configured for full FCrDNS compliance.
• HELO/Received headers: While not guaranteed, the HELO command in the SMTP transaction and 'Received' headers (where intermediate servers perform a reverse DNS lookup) are typical places to find the actual hostname of the sending server.
• Automated tools: Experts recommend utilizing or developing specialized tools that can automate the FCrDNS check process, especially for large lists of IPs or hostnames, to save time and ensure accuracy.
• Proactive approach: As mail servers increasingly rely on FCrDNS for spam filtering and reputation, ensuring your sending infrastructure passes this check is a proactive step towards better email deliverability.

Key findings

• Mandatory for compliance: Mailbox providers, such as Gmail, explicitly require PTR records as part of their email sender guidelines, especially for high-volume senders, to ensure authenticity.
• Verification standard: PTR records facilitate reverse DNS lookups, allowing receiving servers to verify that the sending IP address matches the hostname declared by the sending mail server. This is a crucial step in email authentication.
• Combating spoofing: FCrDNS helps to validate the legitimacy of email senders by confirming that the IP address and hostname are consistent, making it harder for spammers and phishers to spoof legitimate domains.
• Tools and scripts: Documentation often includes examples and references to tools (e.g., nmap scripts, online DNS checkers) that can be used to perform FCrDNS checks, demonstrating practical application.

Key considerations

• ISP responsibility: The PTR record must be created or updated by the organization that owns the IP address space, typically the Internet Service Provider (ISP) or hosting provider, rather than directly by the domain owner.
• Consistency of records: For optimal deliverability, the hostname in the PTR record should align with the hostname used in the mail server's HELO/EHLO greeting. This provides a clear, consistent identity for the sending server.
• Impact on blocklists: Missing or misconfigured PTR records can negatively impact sender reputation and may lead to an IP address being listed on email blocklists or blacklists, hindering email delivery.
• Troubleshooting: If emails are experiencing deliverability issues due to PTR record errors, documentation often advises contacting the IP address owner to create or update the reverse DNS record.