Is using different host name subdomains pointing to IPs a potential flag for email deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 17 Apr 2025
Updated 17 Aug 2025
7 min read
Email deliverability is a complex landscape, and understanding how different DNS configurations impact your sending reputation is crucial. A common question that arises for senders managing multiple email streams involves hostnames and IP addresses: does using different hostname subdomains, each pointing to unique IPs, inherently flag your emails for deliverability issues?
In short, no. This setup is a standard and often recommended practice, especially for high-volume senders or those who segment their email traffic. The key lies not in the mere fact of using different subdomains and IPs, but in the meticulous configuration and management of the underlying DNS records for each sending entity. When properly implemented, this strategy can actually enhance your overall email deliverability and sender reputation.
The perception of this configuration as a potential flag often stems from a misunderstanding of how email authentication and reputation systems work. Each sending element, whether it's a domain, subdomain, or IP, contributes to your sender identity and reputation, and isolating these elements can be a powerful tool for maintaining good inbox placement. Let's dive deeper into the technicalities and best practices.
Understanding hostname subdomains and IP addresses
A hostname, in the context of email sending, refers to the name given to a device connected to a network, often a mail server. A subdomain is a division of your main domain, such as mail.yourdomain.com. Subdomains can have their own DNS records, allowing them to point to different IP addresses than the root domain. This separation is fundamental to segmenting email traffic. The Domain Name System (DNS) translates these human-readable domain names into the numerical IP addresses that computers use to locate services.
When you configure a subdomain to point to a specific IP address, you are typically setting up an A record (for IPv4) or AAAA record (for IPv6) in your DNS. This record tells the internet where to find the server associated with that subdomain. For example, if you want email.yourdomain.com to use a dedicated IP for sending, you would create a record like this:
Example A record for a sending subdomainDNS
email.yourdomain.com. IN A 192.0.2.1
This setup allows you to isolate your email traffic. For instance, you could use transactional.yourdomain.com for order confirmations and marketing.yourdomain.com for newsletters, each with its own IP and reputation. This is a common and effective strategy for segmenting email streams for better deliverability.
The role of DNS records in deliverability
The critical element in ensuring these distinct subdomains don't become a flag is correct DNS configuration. Beyond the basic A/AAAA records, you must ensure proper reverse DNS (PTR) records are in place for each sending IP. A PTR record translates an IP address back to a hostname, providing a crucial layer of verification for recipient servers.
Additionally, each sending subdomain must have its own SPF, DKIM, and DMARC records correctly configured and aligned. These email authentication protocols are vital for proving that your emails are legitimate and haven't been tampered with. Without proper alignment, even a well-intentioned setup can lead to deliverability issues.
DMARC alignment
When using multiple subdomains, particularly if the From address varies across them, pay close attention to DMARC alignment. You may need to use a relaxed alignment policy (rather than strict) to ensure your emails pass DMARC checks, especially if the organizational domain for the SPF or DKIM identifier differs from the From domain.
Incorrect or missing DNS records, such as an absent PTR record or misconfigured SPF, DKIM, and DMARC, are major red flags for internet service providers (ISPs) and can lead to emails being sent to spam folders or even being rejected outright. Ensuring every component of your sending infrastructure is meticulously configured is paramount.
Impact on sender reputation
Each subdomain you use effectively builds its own sender reputation. This is a key advantage, as it allows you to isolate different email streams and their associated risks. For example, if your marketing emails experience a high complaint rate, it will primarily affect the reputation of your marketing subdomain and its associated IP, rather than your main domain or your critical transactional email stream. Using subdomains helps protect your primary domain reputation.
However, it's important to understand that while subdomains provide a degree of isolation, severe issues on a subdomain can still have ripple effects. If a subdomain is heavily abused and ends up on a major email blacklist (or blocklist), some recipient servers may also penalize other subdomains or even the root domain. This is why careful management of each sending entity is non-negotiable. Even when isolating, it's possible that if the parent domain is blocked, subdomains may be affected, as noted in deliverability guides.
Good isolation
Reputation isolation: Different types of emails (e.g., transactional, promotional) are sent from distinct subdomains and IP addresses. This minimizes the risk of one stream negatively impacting another.
Dedicated resources: Each subdomain has its own dedicated IP(s) and proper DNS records (SPF, DKIM, DMARC, PTR) aligned correctly. This ensures a clear, verifiable sender identity.
Proactive monitoring: Consistent monitoring of deliverability metrics, complaint rates, and blocklist status for each subdomain is performed, allowing for quick intervention if issues arise.
Bad isolation
Reputation spill-over: High spam complaints or low engagement on one subdomain's IP may still impact the deliverability of the root domain or other subdomains, especially if ISPs link them.
Configuration errors: Missing or misconfigured DNS records (e.g., PTR records not matching, incorrect SPF/DKIM) can lead to authentication failures and emails being sent to spam or rejected.
Lack of monitoring: Without regular monitoring, poor sender reputation on a subdomain can go unnoticed, leading to widespread deliverability problems before they are identified.
When an IP address or domain is placed on an email blocklist (or blacklist), it means that mail servers are likely to reject or quarantine emails originating from that source. Using different IPs for subdomains means that if one IP gets blocklisted, it doesn't necessarily take down your entire sending operation. However, how your address ends up on a blocklist is often tied to poor sending practices.
Best practices for managing multiple sending entities
To leverage the benefits of using different hostname subdomains and IP addresses without raising red flags, a strategic approach is necessary. First, carefully consider why you are segmenting your email streams. Clear segmentation for different types of mail (e.g., transactional, marketing, notifications) is a valid reason. This allows each stream to build its own independent reputation based on its specific sending practices and engagement.
When introducing new subdomains or IP addresses, a proper warm-up process is essential. This involves gradually increasing your sending volume from the new IP over time, allowing ISPs to establish a positive reputation for the new sender. Rushing this process can lead to immediate deliverability issues.
Key considerations for new subdomains
Monitor actively: Keep a close eye on your sender reputation using tools like Google Postmaster Tools and other deliverability dashboards.
Consistent send rates: Maintain consistent sending volumes and frequencies for each subdomain and IP. Erratic sending patterns can appear suspicious. Some providers like Brevo advise using different subdomains for each dedicated IP.
Dedicated IP addresses: Use dedicated IPs whenever possible for higher sending volumes, as this improves email reliability.
Ultimately, the strategic use of different hostname subdomains pointing to different IPs is a sophisticated approach to email deliverability. It's not a red flag in itself, but rather an advanced technique that requires careful planning, precise technical configuration, and ongoing monitoring to succeed.
Views from the trenches
Best practices
Always ensure a clear, strategic reason for using multiple subdomains for email sending.
Set up correct reverse DNS (PTR) records for every sending IP, pointing to the specific hostname.
Implement and align SPF, DKIM, and DMARC for each subdomain to authenticate your emails.
Warm up new subdomains and IPs gradually to build a positive sending reputation.
Continuously monitor deliverability metrics and blocklist status for each email stream.
Common pitfalls
Misconfiguring DNS records, especially PTR, SPF, DKIM, and DMARC for subdomains.
Assuming that subdomains completely isolate reputation, ignoring potential ripple effects.
Rushing the warm-up process for new subdomains or IPs, leading to immediate spam flagging.
Lacking a clear strategy for subdomain use, leading to unnecessary complexity and potential issues.
Not actively monitoring deliverability, allowing problems to escalate unnoticed across streams.
Expert tips
For optimal reputation management, separate transactional and marketing email streams with different subdomains and dedicated IPs.
If using multiple subdomains, particularly in the From: header, ensure DMARC alignment is handled correctly, potentially with a relaxed policy.
Regularly audit your DNS settings for all sending subdomains to catch any misconfigurations early.
Understand that while subdomains provide isolation, a severely compromised subdomain can still impact the parent domain's overall perception.
Always question the 'why' behind using subdomains for email; ensure it serves a genuine deliverability or segmentation purpose.
Expert view
Expert from Email Geeks says using multiple domains on a single IP is generally fine, as evidenced by shared IP pools, provided the EHLO (sending server's hostname) has a valid PTR record pointing back to it.
2024-02-29 - Email Geeks
Expert view
Expert from Email Geeks notes that using subdomains in the RFC 822 From header can work, but advises relaxing DMARC alignment requirements if the setup is complex.
2024-02-29 - Email Geeks
Strategic subdomain use for improved deliverability
Using different hostname subdomains pointing to unique IP addresses is not a red flag for email deliverability. On the contrary, when executed correctly, it's a powerful strategy that can improve your email program's resilience and inbox placement. The key is in the details of your DNS configuration and ongoing reputation management.
Each subdomain and its associated IP should be treated as a distinct sender with its own reputation. This means diligently setting up SPF, DKIM, and DMARC records, ensuring proper PTR records are in place, and consistently monitoring deliverability metrics.
By segmenting your email traffic and maintaining a healthy sending reputation for each stream, you can protect your primary domain, prevent blocklist issues from cascading across your entire email program, and ultimately achieve higher inbox placement rates. The strategic use of subdomains is a sign of a sophisticated and proactive approach to email security and deliverability.
Monitor actively: Keep a close eye on your sender reputation using tools like Google Postmaster Tools and other deliverability dashboards.
Consistent send rates: Maintain consistent sending volumes and frequencies for each subdomain and IP. Erratic sending patterns can appear suspicious. Some providers like Brevo advise using different subdomains for each dedicated IP.
Dedicated IP addresses: Use dedicated IPs whenever possible for higher sending volumes, as this improves email reliability.
