Should I use multiple subdomains for different email streams or a single stream?
Michael Ko
Co-founder & CEO, Suped
Published 12 Aug 2025
Updated 19 Aug 2025
6 min read
Deciding on an email domain strategy, especially for high-volume senders, involves navigating complex considerations around deliverability and sender reputation. A common question I encounter is whether to use multiple subdomains for different email streams or stick to a single, consolidated stream. This choice significantly impacts how your emails perform and how effectively you can manage potential issues.
Many organizations start by sending all their emails from their root domain, like company.com. While convenient initially, this approach can quickly become risky as email volumes grow and stream types diversify. If a promotional campaign, for instance, triggers spam complaints, it can negatively affect the deliverability of critical transactional emails sent from the same domain.
The alternative, using subdomains, offers a way to segment and protect your sending reputation. For example, you might use user.company.com for user profiles and one-time passwords, click.company.com for marketing emails, and alerts.company.com for critical notifications. This segmentation allows Internet Service Providers (ISPs) to evaluate each email stream independently, reducing the risk of a single poor-performing stream impacting others.
Why separate your email streams?
Using subdomains for different email streams is generally considered a best practice for improving email deliverability and maintaining a strong sender reputation. Each subdomain develops its own reputation based on the engagement metrics and complaint rates associated with the email sent from it.
Separating transactional and promotional emails onto different subdomains is a common and highly recommended strategy. Transactional emails, like password resets or order confirmations, typically have high open and click-through rates because recipients expect them. Promotional emails, on the other hand, might have lower engagement and higher complaint rates. By isolating these, you protect the crucial transactional stream from being impacted by issues on the marketing side.
Email subdomains are not strictly necessary for every sender, but they offer significant advantages. For organizations sending large volumes or diverse types of emails, the benefits of reputation isolation, clearer analytics, and simplified troubleshooting often outweigh the slight increase in DNS management complexity. This segregation makes it easier to pinpoint the source of deliverability issues, whether it is a specific campaign or a certain type of email.
Implementing email subdomains also allows for distinct authentication records (SPF, DKIM, DMARC) for each stream. This means that if one subdomain's authentication records are misconfigured or compromised, it is less likely to affect the deliverability of emails from your other subdomains. It also provides flexibility to tailor your authentication policies based on the risk profile of each stream.
Multiple subdomains for a single stream?
While separating different *types* of email streams is generally beneficial, the idea of breaking down a *single* large email stream, like critical alerts (26 million per day), into multiple subdomains (e.g., alerts1.company.com, alerts2.company.com, etc.), introduces more nuanced considerations.
Benefits of multiple subdomains per stream
Granular insight: Provides detailed insights into deliverability and reputation issues specific to each subdomain, making troubleshooting easier.
Traffic shifting: In theory, allows you to move traffic to other subdomains if one gets blocked or blacklisted, mitigating downtime. This relies on existing reputation on other subdomains.
Snowshoeing risk: ISPs might perceive using many subdomains for the same mail type as an attempt to evade blocklists (also known as blacklists) or reputation filters, similar to an old spamming technique called "snowshoeing." This can lead to broader blocks, even at the parent domain level.
Reputation dilution: Spreading volume across too many subdomains can dilute the reputation of each, making it harder to build strong trust with ISPs, especially for smaller volumes per subdomain. ISPs prefer consistent, high-volume sending from fewer, well-maintained sender identities.
Management complexity: More subdomains mean more DNS records to manage (SPF, DKIM, DMARC, custom tracking), increasing operational overhead and potential for errors. This complexity scales with volume, making it significant for 26 million sends per day.
Warm-up challenges: If you need to shift a large volume of traffic to a new or less-used subdomain, it will require a careful warm-up phase to build its reputation. Failing to do so can trigger spam filters.
My advice leans towards separating *different types* of email streams into their own subdomains, but not necessarily splitting a *single type* of email, especially high-volume ones, across multiple subdomains. The reasoning is that deliverability problems tend to follow the underlying mail stream's quality, not just the specific subdomain it is sent from. If your alerts are generating issues, splitting them into three subdomains won't fix the root cause and may just spread the problem.
For a large volume like 26 million alerts per day, a single, well-managed alerts.company.com subdomain with its own dedicated IP pool can build a very stable and positive reputation. ISPs often favor consistent, high-volume sending from a single, reputable source. The focus should be on ensuring the quality and engagement of that single stream rather than trying to create multiple fallback options.
Strategic considerations for high-volume senders
Even with a single subdomain for your alerts, you can still gain significant insight and maintain control. Monitoring your DMARC reports (I recommend using a DMARC monitoring service), sender reputation tools, and bounce rates will provide the necessary data to troubleshoot any issues efficiently. If a problem arises, it will be isolated to that specific stream, allowing you to address it directly without affecting your other email types.
High-volume senders, like those sending millions of emails daily, often consolidate their highest-volume, most critical streams onto a single, dedicated subdomain and IP pool. This simplifies management and allows them to build a robust, consistent sender reputation. The risk of diluting that reputation or accidentally triggering spam filters due to a snowshoeing perception outweighs the perceived benefit of having multiple subdomains to hop between in case of issues.
Smart subdomain naming conventions
When naming your subdomains, aim for clarity and recipient recognition. While sequential names like a.domain.com and b.domain.com work, more descriptive names like alerts.company.com or marketing.company.com are preferable. They clearly communicate the purpose of the email stream to both recipients and ISPs, aiding in trust and deliverability. Consider how many you need, and consult our guide on how many subdomains to create.
Final recommendation
In conclusion, segmenting your email streams with subdomains is a powerful strategy for enhancing email deliverability and protecting your sender reputation. For instance, having distinct subdomains for transactional, marketing, and alert emails is a sound practice. This approach isolates reputation, simplifies troubleshooting, and provides clear insights into the performance of each stream.
However, extending this segmentation to create multiple subdomains for a *single* high-volume stream, such as breaking down alerts into alerts1, alerts2, and alerts3, introduces more risks than benefits. It can lead to reputation dilution, increased management complexity, and a higher chance of being flagged for snowshoe spamming, where a problem on one subdomain could still lead to broader issues for the parent domain. Your primary goal should be to maintain a pristine reputation on each dedicated subdomain through consistent, high-quality sending, rather than attempting to diversify for hypothetical failover.
For specific deliverability challenges or very unique sending patterns, consulting with a deliverability expert can provide tailored guidance. While the idea of having multiple subdomains to subdomain hop might seem appealing for mitigating downtime, it often creates more problems than it solves in the long run. Focus on the underlying quality of your mail streams and the proper configuration of your DNS records, including a robust DMARC, SPF, and DKIM setup, to ensure optimal inbox placement.
Views from the trenches
Best practices
Always separate different types of email streams, such as marketing and transactional, onto distinct subdomains to isolate reputation.
For very high-volume, homogeneous email streams (e.g., alerts), concentrate sending on a single, dedicated subdomain with a strong reputation.
Choose clear, recognizable subdomain names that indicate their purpose to both recipients and ISPs.
Common pitfalls
Splitting a single email stream across many subdomains, which can dilute reputation and be perceived as spammy (snowshoeing).
Attempting to "subdomain hop" to evade blocklists instead of addressing the root cause of deliverability problems.
Failing to warm up new subdomains or drastically increasing volume on existing ones without proper ramp-up.
Expert tips
The volume thresholds for subdomain use are not static. What works for 10,000 emails per month differs from 26 million daily.
Recipient engagement with a specific stream is paramount; a good reputation on one subdomain won't automatically transfer to another.
Consider the long-term management overhead versus the perceived short-term benefits of excessive subdomain fragmentation.
Expert view
Expert from Email Geeks says that having many subdomains from an IP or set of IPs primarily looks like a shared pool. Content similarity for transactional mail is less of a concern, and cross-domain reputation bleed isn't guaranteed if best practices are followed. Issues will still happen, but they will be isolated to a particular domain, which is a lifesaver for troubleshooting.
2022-03-23 - Email Geeks
Marketer view
Marketer from Email Geeks says that using many subdomains for the same mail type could appear suspicious, similar to snowshoeing IPs. If one domain gets blocked, moving traffic to another might just burn that one too, as each subdomain has its own reputation.
