Should I use subdomains for transactional and promotional emails to protect my main domain reputation?
Michael Ko
Co-founder & CEO, Suped
Published 26 May 2025
Updated 16 Aug 2025
8 min read
The question of whether to use subdomains for different email types, especially transactional versus promotional messages, is common among businesses as they scale. Many believe that segmenting email streams onto subdomains inherently safeguards their main domain's reputation. The idea is that if your promotional emails, which often carry higher risk, encounter deliverability issues like spam complaints or blocklistings, the negative impact will be contained to that specific subdomain, leaving your primary domain for critical transactional emails (like password resets or order confirmations) unaffected.
While this strategy has merit, the reality is more nuanced. Mailbox providers (MBPs) are increasingly sophisticated in how they assess sender reputation, often looking beyond just the immediate sending domain. However, strategically using subdomains can still be a powerful tool for maintaining strong email deliverability, provided it's implemented with a clear understanding of its benefits and limitations.
Understanding email domain reputation
Domain reputation is a critical factor in email deliverability, influencing whether your emails land in the inbox or the spam folder. It's essentially a score assigned by mailbox providers based on your sending history, including factors like bounce rates, spam complaint rates, engagement, and whether your domain appears on any email blacklist or blocklist (or blocklist).
Transactional emails, like password resets or purchase confirmations, are typically highly anticipated and have high engagement rates. Recipients expect these emails, so they are less likely to mark them as spam, contributing positively to your sender reputation. Promotional emails, on the other hand, are often sent in bulk, might be less expected, and can have lower engagement, leading to higher spam complaint rates or unsubscribes. This difference in user interaction means each email type inherently carries a different risk profile for your domain's sending reputation.
Mailbox providers constantly monitor these metrics at various levels, including the IP address, the sending domain, and even the parent domain. A poor reputation on one sending entity can, in some cases, negatively influence the reputation of related entities, such as the main domain. Understanding this hierarchy is key to making informed decisions about your email infrastructure.
The role of subdomains in email strategy
An email subdomain is a prefix added to your main domain (e.g., mail.yourdomain.com or marketing.yourdomain.com). It creates a distinct identity for specific email sending purposes, allowing you to manage different types of email traffic separately. This isolation is the primary perceived benefit. For instance, if your promotional emails sent from marketing.yourdomain.com experience high spam complaints, the reputation hit would ideally be confined to that subdomain, leaving yourdomain.com relatively untouched for your crucial transactional messages.
The main reason for using subdomains is indeed to isolate the reputation of different email streams. While some older reputation systems might have been more easily siloed by subdomains, modern systems are more interconnected. If a subdomain engages in highly abusive sending practices, it can still signal negative reputation to the parent domain. However, for most legitimate sending, separating high-volume or higher-risk promotional emails from low-volume, high-engagement transactional emails onto different subdomains is a common and often effective strategy.
This approach allows you to build distinct reputations for each sending purpose. For instance, your transactional subdomain can cultivate a pristine reputation due to high engagement and low complaints, ensuring critical emails always land in the inbox. Simultaneously, your promotional subdomain can absorb the occasional bounce or complaint without jeopardizing the core communication channels of your business. This concept is highlighted in various industry best practices, including a document by M3AAWG on sending domain best common practices.
Transactional emails
Purpose: Critical, timely notifications like password resets, order confirmations, shipping updates.
Engagement: High, as recipients expect and need these emails.
Deliverability impact: Poor deliverability can severely impact user experience and business operations.
Mailgun advises separating these streams to mitigate risk and optimize inbox placement, as different types of emails can have distinct deliverability profiles.
Promotional emails
Purpose: Marketing campaigns, newsletters, special offers, bulk communications.
Engagement: Variable, potentially lower than transactional, leading to higher complaints.
Deliverability impact: Issues are less critical to core business function but can harm marketing ROI.
ActiveCampaign emphasizes that using subdomains can help minimize potential domain reputation problems for marketing emails, ensuring they reach their intended destination. Their guide on subdomains and deliverability covers this.
Benefits and considerations of using subdomains
The primary benefit of using subdomains is the ability to manage and protect your main domain's reputation. By separating email streams, you reduce the risk of a problematic marketing campaign (e.g., one with a high spam complaint rate) from negatively affecting the deliverability of your essential transactional communications. This segmentation can lead to better inbox placement for your critical emails, as their dedicated subdomain builds a strong, clean reputation.
Another advantage is enhanced tracking and analytics. With separate subdomains, you can more easily monitor the performance of each email stream independently. This granular data helps identify specific deliverability issues and optimize your sending practices for each type of email. For example, you can track the inbox placement rates and engagement metrics for transactional.yourdomain.com versus marketing.yourdomain.com, allowing for targeted improvements. This also gives you flexibility to quickly adjust sending strategies if one stream starts to underperform.
Implementing subdomains requires proper email authentication records, including SPF, DKIM, and DMARC, for each one. This ensures that emails sent from each subdomain are properly authenticated and trusted by receiving mail servers. For example, a DKIM record for a transactional subdomain might look like this:
This setup allows you to align your email authentication with the specific subdomain, providing clear signals to mailbox providers about the sender's legitimacy. We have a simple guide to DMARC, SPF, and DKIM if you need more information.
Beware of false security
While subdomains offer a layer of protection, it's crucial to understand they don't provide complete isolation from your main domain. If one subdomain engages in severely abusive or spammy sending behavior, it can still negatively impact the reputation of your entire parent domain. Mailbox providers are increasingly sophisticated and can correlate activity across related domains and subdomains. Always adhere to best practices across all your sending entities, regardless of subdomain separation.
When to implement subdomains (and when not to)
You should consider using subdomains when you have distinct types of email traffic with different sending volumes and engagement expectations. For example, if you send thousands of marketing emails weekly but only hundreds of transactional emails, separating them makes sense. This strategy is particularly valuable when you're scaling operations, as it allows for more precise reputation management and better insight into the performance of each email stream. You might also want to read about using separate IPs or domains for transactional vs marketing emails.
Conversely, if your email volume is low across all types, or if your transactional and promotional emails consistently achieve high engagement and low complaint rates, the complexity of setting up and managing multiple subdomains might outweigh the benefits. In such cases, a single sending domain, carefully managed, might be sufficient. The key is to assess your specific sending patterns and risk tolerance. It's often said, if what you're currently doing is working well and you're not experiencing deliverability issues, there might not be an immediate need for a significant infrastructure overhaul.
Ultimately, the decision to use subdomains should be based on a strategic assessment of your email program's needs and risks. For many businesses, particularly those with diverse email types and growing sending volumes, separating transactional and promotional emails onto dedicated subdomains is a sound practice that can enhance overall email deliverability and protect your main domain's reputation. This is especially true as new email sender requirements, like those from Google and Yahoo, become stricter.
Strategic subdomain usage
The strategic use of subdomains can significantly contribute to better email deliverability and reputation management. By segmenting your email traffic, you create more predictable sending environments for your most critical communications while allowing for more flexibility with high-volume or promotional campaigns. This approach helps to build and maintain trust with mailbox providers, ensuring your messages consistently reach the inbox.
Remember, the goal is to optimize for deliverability, not simply to add subdomains for the sake of it. Regular monitoring of your email performance metrics, including open rates, click-through rates, bounce rates, and spam complaints, across all your sending domains and subdomains is crucial for ongoing success. Adapting your strategy based on these insights will ensure your email program remains healthy and effective.
Views from the trenches
Best practices
Maintain separate fully authenticated subdomains, each with its own DKIM and SPF records, for different email streams to manage reputation.
Segment email traffic based on volume and engagement to ensure critical emails are not impacted by promotional campaign issues.
Continuously monitor specific deliverability metrics for each subdomain to identify and address problems proactively.
Common pitfalls
Assuming subdomains offer absolute reputation isolation, as severe issues on one can still affect the parent domain.
Implementing subdomains without a tangible need, adding unnecessary complexity to your email setup.
Failing to set up proper email authentication (SPF, DKIM, DMARC) for each new subdomain, hindering deliverability.
Expert tips
If your current setup is working well without deliverability issues, avoid making unnecessary changes.
For aggressive cold outreach, consider using a wholly separate domain rather than a subdomain of your main brand.
Focus on sending wanted mail to opted-in recipients to significantly reduce the chances of domain reputation problems.
Expert view
Expert from Email Geeks says that if one subdomain sends spam, the parent domain's reputation and other subdomains might still be impacted, as modern reputation systems are sophisticated.
2022-09-21 - Email Geeks
Expert view
Expert from Email Geeks suggests setting up authentication with specific DKIM signatures for subdomains, like `d=promotions.company.com`, which helps segregate streams without altering the visible 'From' address.
Mailgun advises separating these streams to mitigate risk and optimize inbox placement, as different types of emails can have distinct deliverability profiles.
ActiveCampaign emphasizes that using subdomains can help minimize potential domain reputation problems for marketing emails, ensuring they reach their intended destination. Their guide on subdomains and deliverability covers this.
