Summary
Configuring reverse DNS (rDNS) for email sending when using multiple IP addresses can seem complex, but the core principle is that each unique IP address requires its own distinct PTR (pointer) record. While you might want all IP addresses to point back to a single main sending domain, the best practice involves creating specific hostnames (e.g., mail1.yourdomain.com, mail2.yourdomain.com) for each IP. Each of these hostnames should then have its rDNS record (PTR record) configured by your IP provider (typically your ESP or hosting provider) to point back to that unique hostname, and an A record on your end to resolve the hostname to the IP. These specific hostnames should, however, be within the same overarching domain space as your primary sending domain to maintain alignment for mailbox providers. For further reading on related topics, you might want to explore best practices for reverse DNS resolution or email service provider guidelines for HELO, rDNS, and SPF. Understanding how rDNS works can significantly improve your email deliverability rates.
Key findings
- Unique Hostnames: For each dedicated IP address, create a unique hostname (e.g., mta1.yourdomain.com, mta2.yourdomain.com) rather than trying to point one hostname to multiple IPs for email sending.
- One-to-One Mapping: The fundamental rule for rDNS is a one-to-one mapping, where one IP address corresponds to one PTR record, which in turn points to one hostname.
- Domain Alignment: The hostnames used for rDNS, even if they have prefixes, should align with your main sending domain (e.g., mta1.ml.hostname.com is aligned with ml.hostname.com). This alignment is crucial for mailbox providers to trust your sending.
- A Record Requirement: Before your provider can set up the PTR record, you typically need to create an A record in your DNS zone that maps your chosen hostname to the specific IP address.
- EHLO Value Consideration: It is beneficial to also set up a simple SPF TXT record for each of these hostnames. This is because some mail transfer agents (MTAs) may perform SPF lookups on the EHLO value, which often matches the hostname. For example, a record might look like "v=spf1 ip4:111.222.111.222 -all".
- Checking Headers: Always verify what your sending machine is actually using in its EHLO value by examining the full email headers of a test message. This helps ensure your DNS configuration aligns with your server's behavior. More on this can be found in a detailed guide to setting up rDNS.
Key considerations
- Provider Support: Your Email Service Provider (ESP) or hosting provider is responsible for configuring the PTR records on the IP addresses they allocate to you. Confirm their specific requirements and procedures.
- DNS Propagation: DNS changes, including A records and PTR records, can take time to propagate across the internet. Plan accordingly and allow sufficient time for these changes to take effect.
- Deliverability Impact: Proper rDNS configuration is a critical factor for email deliverability and sender reputation. Mailbox providers often perform rDNS lookups as part of their anti-spam checks.
- Consistency: Ensure consistency between your EHLO/HELO banner, your PTR record, and your sending domain. While not always an exact match, they should logically belong to the same entity or domain space. For more information, see how to maintain dedicated IP reputation.
- Subdomain Strategy: Using distinct subdomains like mta1.ml.hostname.com for rDNS allows for clear, verifiable mapping and helps avoid potential flagging by stricter anti-spam tools.
What email marketers say
Email marketers often encounter challenges when scaling their email infrastructure, particularly when moving from a single IP setup to multiple dedicated IP addresses. The process of configuring rDNS (reverse DNS) for these additional IPs can introduce confusion, as traditional DNS A records map a hostname to an IP, but rDNS maps an IP back to a hostname. Marketers frequently wonder if their preferred sending domain can be the direct rDNS target for all IPs, or if unique hostnames are required for each IP address, and if so, how this affects deliverability. The community largely agrees that while DNS allows flexibility, email deliverability best practices lean towards more precise configurations to ensure mailbox providers can easily verify sender identity, impacting inbox placement and sender reputation. Learning how to run an email deliverability test is a crucial step after configuring rDNS.
Key opinions
- Prefixing Hostnames: Many marketers find success by using prefixed hostnames (e.g., mail1.example.com, mail2.example.com) for each IP address, which simplifies rDNS setup with ESPs like Mailgun.
- Alignment is Key: The critical factor for mailbox providers is that the PTR record resolves to a hostname within the same overall domain space as the authenticated sending domain, not necessarily an exact match.
- Avoid One A Record to Multiple IPs: While DNS technically allows one A record to point to multiple IP addresses, this setup is generally not recommended for email sending due to potential deliverability issues flagged by some older or stricter anti-spam tools.
- Simplified Naming: Marketers often adopt simple, sequential naming patterns like mailX or mtaX to manage multiple sending IPs efficiently.
Key considerations
- ISP Specifics: Some smaller ISPs (particularly in Europe) have very strict DNS checks, making precise rDNS configuration crucial to avoid deliverability problems. Staying on top of current reverse DNS white labeling practices is important.
- Authenticated Domain vs. PTR: Marketers often initially assume the PTR record should point directly to their main authenticated (sub)domain. However, the use of unique hostnames for each IP, which are themselves subdomains of the main domain, is the standard and accepted practice. This is related to how you map dedicated IPs to subdomains.
- SPF for EHLO: Beyond the primary SPF record for your sending domain, it is a good practice to create a simple SPF TXT record for each of your rDNS hostnames (e.g., "v=spf1 ip4:your.ip.address -all") as a secondary validation point for receiving servers.
- Verification Process: Always verify that the rDNS records have propagated correctly and that the IP addresses resolve back to the expected hostnames. This can be done through various online DNS lookup tools.
Marketer view
Email marketer from Email Geeks explains that they were confused about rDNS when trying to send from multiple IP addresses. They initially thought they could not point the same hostname to multiple IP addresses with different A records, leading to uncertainty about how to properly set up PTR records for their authenticated subdomain.
Marketer view
Email marketer from Spiceworks Community observes that you generally have only one rDNS entry per IP address. They recommend picking a primary domain and using that for the rDNS, then relying on SPF records for additional verification.
What the experts say
Email deliverability experts consistently advise that while the DNS system allows flexibility, email sending protocols and receiving mailbox providers (MBPs) require a more stringent approach to rDNS configuration, especially with multiple IP addresses. The consensus is that each dedicated IP address should have a unique, resolvable PTR record that points to a specific hostname. This hostname, in turn, should have a corresponding A record pointing back to that IP. Furthermore, experts highlight the importance of ensuring these hostnames fall within the same broader domain as your primary sending domain, even if they have prefixes. This setup helps maintain trust and avoids potential blocklisting or filtering by recipient servers. It's a key part of maintaining good domain reputation.
Key opinions
- Distinct Hostnames for Each IP: Experts recommend setting up unique hostnames for each IP address, such as mta1.yourdomain.com and mta2.yourdomain.com, to ensure proper rDNS resolution.
- No Mismatch is Key: A crucial point is that there should be no "mismatch" between the IP, its PTR record, and the hostname, meaning a simple, direct, one-to-one resolution is preferred for email.
- Domain Space Consistency: While an exact FQDN match might not always be necessary, the rDNS hostname should be within the same domain space as the sending domain to achieve alignment and avoid flagging by stricter receiving servers.
- SPF for EHLO Best Practice: It is a recommended best practice to add a simple SPF TXT record for each rDNS hostname, as some MTAs will check the EHLO value during connection, which should typically match the hostname.
Key considerations
- EHLO Verification: Always verify the actual EHLO value your mail server is using by inspecting email headers. This ensures that your DNS configurations (A record and PTR record) align with your server's sending behavior. This impacts overall email deliverability.
- ISP Strictness: Be aware that certain smaller ISPs and anti-spam tools might have very strict or even "silly" requirements regarding rDNS, making proper configuration critical to avoid deliverability issues.
- Header Analysis: Detailed analysis of Received headers in sent emails is essential. This helps in understanding how the mail server identifies itself (EHLO) and how its IP resolves via rDNS. You can learn more about troubleshooting DMARC reports which often rely on these headers.
- Forward and Reverse Check: A robust rDNS setup ensures that an IP address resolves to a hostname (PTR record), and that hostname then resolves back to the same IP address (A record). This bidirectional consistency is a strong signal of legitimacy.
Expert view
Expert from Email Geeks (U02KPM4GEDB) advises that using distinct hostnames such as mta.ml.hostname.com and mta2.ml.hostname.com for multiple IP addresses is the common and expected approach, similar to practices at large organizations like Salesforce.
Expert view
Expert from Spam Resource explains that while you can technically have multiple IP addresses for the same hostname in general DNS, for email, it's highly recommended to have a one-to-one mapping between an IP address and its PTR record for optimal deliverability.
What the documentation says
Technical documentation for DNS and email sending consistently outlines the requirements for reverse DNS configuration, particularly for multiple IP addresses. The standard dictates that each IP address used for sending mail must have a corresponding PTR record, enabling reverse lookup to a hostname. While the PTR record is managed by the IP owner (e.g., your ESP or hosting provider), you, as the domain owner, are responsible for creating the forward A record that maps the chosen hostname to the IP. This creates the necessary bidirectional resolution that receiving mail servers expect for verification. The documentation stresses the importance of this alignment for anti-spam filters and overall email deliverability. For more on the specifics, check out articles on DMARC, SPF, and DKIM, as these authentication methods often rely on correct rDNS.
Key findings
- PTR Records and IP Ownership: PTR records, which are essential for rDNS, must be set by the entity that controls the IP address space (i.e., your ISP or ESP). You must typically provide them with the hostname you wish to associate with your IP.
- Bi-directional Mapping: For proper email validation, the rDNS (PTR record) should resolve an IP to a hostname, and a forward DNS (A record) lookup of that hostname should resolve back to the same IP. This forward-confirmed reverse DNS (FCrDNS) is a strong trust signal.
- Subdomain Use: Documentation often implies that using a subdomain that logically relates to the sending domain (e.g., mail.yourdomain.com) for the PTR record of an IP is sufficient for alignment, even if multiple IPs use different sub-hostnames within that same domain.
- Importance for Anti-Spam: Missing or misconfigured rDNS is a common reason for emails to be rejected or routed to spam folders, as it's a basic verification check used by most receiving mail servers.
Key considerations
- DNS Zone for PTR: To set up rDNS, a special type of DNS zone called a Reverse DNS zone is required, which is where PTR records reside. Your IP provider manages this zone.
- Consistency with HELO/EHLO: The hostname configured in your rDNS should ideally be the same as the hostname your Mail Transfer Agent (MTA) uses in its HELO or EHLO greeting, as this is often a check performed by receiving servers.
- Impact on Reputation: Correct rDNS contributes positively to your sender reputation and is a foundational element of email authentication alongside SPF and DKIM. Neglecting it can lead to blocklisting (or blacklisting) of your IPs. See our guide on blocklists.
- Pre-allocation Steps: When obtaining new IP addresses for email sending, it is best practice to configure rDNS immediately. This proactive approach helps establish a clean sending reputation from the start. Read more about warming up a new IP.
Technical article
The CloudNS Blog clarifies that reverse DNS (rDNS) requires creating a special type of DNS zone, known as a Reverse DNS zone, as it's the exclusive location where PTR records can exist and function effectively.
Technical article
ISIPP.com documentation advises that when configuring a mail server, it's crucial to select a suitable name and IP address. They stress the importance of adding the rDNS for a new IP address immediately upon allocation to ensure proper setup.
Related resources
5 resources
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