Is it beneficial to use separate subdomains for transactional and promotional emails?

Yes, it is generally recommended. Subdomains allow you to isolate the sending reputation of different email types. If your marketing emails, for example, encounter deliverability issues, your transactional emails sent from a separate subdomain are less likely to be affected, helping maintain their critical inbox placement.

Do I need to warm up a new subdomain before sending large volumes of email?

Yes, any new subdomain used for sending emails will require a warm-up period, similar to warming up a new IP address. This involves gradually increasing your sending volume over time to build a positive reputation with ISPs before sending at full capacity.

How do email aliases differ from subdomains in terms of deliverability?

Email aliases (e.g., `info@yourdomain.com` vs. `support@yourdomain.com`) typically share the same domain reputation, as ISPs primarily track reputation at the domain level. Subdomains (e.g., `marketing.yourdomain.com` vs. `trans.yourdomain.com`) provide true reputation isolation because they are treated as distinct entities.

How does subdomain segmentation protect my main domain reputation?

Effective subdomain segmentation protects your core domain reputation. If one subdomain experiences a blocklist (blacklist) listing or a drop in engagement, the other subdomains and your main domain are less likely to be impacted, preserving your overall sender reputation.

Should I use subdomains to segment email streams for better deliverability and inbox placement? - Sender reputation - Email deliverability - Knowledge base

Deciding whether to use subdomains to segment your email streams for better deliverability and inbox placement is a common question, and one I hear often. The short answer is, it depends on your specific sending practices and goals. While it might seem like a simple solution, there are nuances to consider that impact how Internet Service Providers (ISPs) perceive your sending reputation and, ultimately, where your emails land.

Many senders, particularly those with diverse email programs, find significant advantages in separating their email traffic. This approach allows for a more granular control over sender reputation, potentially protecting critical email streams from issues arising in others. We will explore the benefits and considerations of adopting a subdomain strategy, including how it impacts your sender reputation and the technical aspects of implementation.

The core idea behind segmenting email streams via subdomains is to isolate the reputation of different types of email. For instance, if your marketing emails experience a sudden spike in complaints or bounce rates, you want to ensure that your critical transactional emails, like password resets or order confirmations, are not affected. This isolation helps maintain high inbox placement rates for your most important communications.

The importance of email reputation

Why email reputation matters

Sender reputation is a crucial factor that ISPs use to determine if your emails should be delivered to the inbox, sent to spam, or rejected entirely. This reputation is tied to your sending domain and IP address. A strong, positive sender reputation is built over time through consistent sending of wanted, engaging emails and adherence to email best practices. Conversely, negative signals, like high spam complaints, bounces, or spam trap hits, can quickly damage your reputation.

When all your email types, from transactional to promotional, are sent from the same domain, their reputations are intertwined. This means a drop in engagement or an increase in complaints on your marketing campaigns can negatively impact the deliverability of your transactional emails. This is where subdomain segmentation offers a protective layer. By separating streams, you create distinct reputation profiles, preventing one stream from jeopardizing another.

ISPs (Internet Service Providers) actively monitor these reputations. They prefer that senders use subdomains because it gives them more options for filtering. If your marketing emails, for example, occasionally perform poorly, the ISP can choose to filter only that specific stream, allowing your more important transactional emails to still reach the inbox. This selective filtering is a key benefit of subdomain usage.

Using subdomains effectively is a core strategy for improving deliverability and inbox placement, particularly as email environments evolve with stricter sender requirements from major providers like Gmail and Yahoo. Maintaining a healthy domain reputation is more critical than ever before. We delve into this in detail in our guide to understanding your email domain reputation.

Subdomains vs. email aliases

It's important to distinguish between using subdomains for segmentation and simply using different email aliases (the part before the '@' symbol) on your main domain. For example, sending from transactions@yourdomain.com and marketing@yourdomain.com from the same root domain (yourdomain.com) does not provide the same reputation isolation as using distinct subdomains like transactions.yourdomain.com and marketing.yourdomain.com. ISPs primarily track reputation at the domain level, meaning mumble@yourdomain.com will largely share the same reputation.

The key lies in how DMARC and DKIM records are configured. For reputation segmentation, it is the DKIM `d=` value (the domain signing the email) that is crucial. By using different subdomains for your DKIM signatures, even if the 'From' address appears to be from your main domain, ISPs will treat them as distinct sending entities with separate reputations. This is a subtle but powerful technique for achieving the desired isolation.

Here's a comparison to illustrate the difference:

Email aliases

Appearance: Different local parts (e.g., `sales@`, `support@`) with the same primary domain.
Reputation Impact: Reputation is largely shared across all aliases as they originate from the same domain.
Deliverability Control: Limited ability to isolate deliverability issues to specific email types.

Subdomains

Appearance: Distinct domains (e.g., `email.yourdomain.com`, `news.yourdomain.com`) recognized by ISPs.
Reputation Impact: Each subdomain can build and maintain its own sender reputation, isolating bad sending practices.
Deliverability Control: Allows for targeted deliverability improvements and risk management per stream.

Implementing subdomain segmentation

If you decide to segment your email streams using subdomains, a crucial step is the warming-up process for each new subdomain. Just like a new IP address, a new subdomain starts with no established reputation. You'll need to gradually increase your sending volume from it, starting with your most engaged subscribers, to build a positive sending history with ISPs. Failing to warm up can lead to immediate deliverability issues, including emails landing in spam folders or being blocklisted (blacklisted).

One effective strategy during warming is to use a technique called double DKIM signing. This involves signing your emails with both your existing, warmed-up domain's DKIM and the new subdomain's DKIM. This allows the new subdomain to gradually inherit some of the positive reputation from the established domain while it builds its own. Over time, you can transition to solely using the new subdomain's DKIM signature.

Remember to configure SPF, DKIM, and DMARC records correctly for each new subdomain. These authentication protocols are foundational to email deliverability and are essential for proving to ISPs that you are an authorized sender. Without them, your emails are much more likely to be flagged as suspicious. You can learn more about these in our simple guide to DMARC, SPF, and DKIM.

Here's an example of how you might structure your subdomains:

Email Stream	Example Subdomain	Purpose and Benefit
Transactional Emails	notify.yourdomain.com	High deliverability, critical for user experience, protected from marketing issues.
Promotional/Marketing	marketing.yourdomain.com	Allows for more aggressive sending practices, reputation isolation.
Newsletter/Content	news.yourdomain.com	Separate stream for regular content, maintains engagement-based reputation.
Warm-up Sending	warmup.yourdomain.com	Dedicated subdomain for new IP or platform warm-up. See our article on how to warm up a domain.

Considerations for specific email types

The type of emails you send plays a significant role in your subdomain strategy. Transactional emails, by their nature, are expected and highly anticipated by recipients. They typically have very high engagement rates and low complaint rates. Therefore, dedicating a subdomain for transactional emails helps ensure their critical deliverability and inbox placement.

On the other hand, promotional emails are often unsolicited and can generate higher complaint rates or lower engagement, even if they are legitimate. Sending these from a separate marketing subdomain means that any negative reputation signals associated with promotional activity won't bleed over and affect your transactional stream. This separation is particularly beneficial for senders with large marketing volumes or those engaged in cold outreach.

For senders looking to optimize for inbox tabs, especially within Gmail's categorization, segmenting by subdomain can also be a factor. By maintaining a clean reputation for transactional (or high-engagement) emails on one subdomain and a separate one for marketing, you can influence which tab each type of email lands in. This is a strategic move for businesses aiming for specific inbox placement for different communication purposes. We have a detailed guide on whether you should use separate email subdomains for transactional and promotional emails.

Views from the trenches

Best practices

Always warm up new subdomains carefully, gradually increasing volume and engagement to build reputation.

Use distinct subdomains for transactional and marketing emails to prevent reputation bleeding between streams.

Ensure SPF, DKIM, and DMARC are correctly configured for all subdomains to authenticate your sending identity.

Regularly monitor the reputation of each subdomain using tools like Google Postmaster Tools.

Align your DKIM `d=` value with the specific subdomain being used for effective reputation isolation.

Common pitfalls

Failing to warm up new subdomains, which can lead to immediate filtering or blacklisting.

Relying solely on email aliases (e.g., info@domain.com) for segmentation, which doesn't provide reputation isolation.

Neglecting to monitor blocklist (blacklist) status for all your sending subdomains.

Increasing sending volume too quickly on new or underperforming subdomains.

Not having consistent DMARC policies across all subdomains, creating potential vulnerabilities.

Expert tips

For optimal reputation separation, ensure both the 'From' address and the DKIM `d=` tag use the specific subdomain.

Consider a phased migration if transitioning an existing high-volume sender to new subdomains.

Prioritize the deliverability of transactional emails as they are often time-sensitive and critical to user experience.

If your overall email program maintains a very high and clean reputation, the benefits of extreme subdomain segmentation might be less pronounced.

If issues arise, always analyze specific bounce messages and consult detailed deliverability reports for affected subdomains.

Expert view

Expert from Email Geeks says the recommendation for separate subdomains is often about protecting some mail streams from bad behavior by other mail streams, but if your program is pretty clean, then you might not need subdomains.

2024-01-29 - Email Geeks

Expert view

Expert from Email Geeks says ISPs prefer subdomains so they can more selectively filter your mail. If your marketing team does iffy things, their poor reputation can just affect marketing mail, allowing transactional mail to still be delivered.

2024-01-29 - Email Geeks

Making an informed decision

Ultimately, the decision to use subdomains for email segmentation hinges on your specific email program, the volume and types of emails you send, and your overall deliverability goals. For many businesses, especially those with high sending volumes or distinct email categories, subdomain segmentation is a valuable strategy for maintaining strong sender reputation and improving inbox placement.

It requires careful planning and execution, particularly regarding warming up new subdomains and ensuring proper authentication. However, the benefits of isolating reputation and ensuring the reliable delivery of critical communications often outweigh the initial effort. By adopting a thoughtful subdomain strategy, you can gain better control over your email deliverability and enhance your overall email marketing effectiveness.