Should I use subdomains for promotional and transactional emails on a shared IP?
Michael Ko
Co-founder & CEO, Suped
Published 29 Apr 2025
Updated 17 Aug 2025
6 min read
When managing email programs, a common question arises regarding the optimal setup for promotional and transactional emails, especially when operating on a shared IP address. It's a balancing act: you want your marketing messages to reach their audience, but you absolutely need your critical transactional emails, like password resets or order confirmations, to land reliably in the inbox.
The core challenge on a shared IP is that your sending reputation is, to some extent, shared with other senders. If one sender on the shared IP engages in problematic practices, it can potentially affect your deliverability. This is where the strategic use of subdomains becomes crucial for safeguarding your various email streams and ensuring each type of email maintains its own distinct reputation.
Understanding shared IP environments
A shared IP address is one that many different senders use. Your email service provider (ESP) assigns you a spot on a server that sends emails for multiple clients. This can be cost-effective and is often the default for smaller senders or those just starting out.
While shared IPs offer convenience, they also come with inherent risks. The sending behavior of other users on that IP directly impacts the collective reputation. If another user sends spam or gets high complaint rates, the IP's overall standing can suffer, which then affects everyone on it, including you. This can lead to your emails being filtered to spam folders or even blocklisted (blacklisted).
This risk is amplified when you send different types of emails from the same domain on a shared IP. Promotional emails, by their nature, often have lower engagement and higher complaint rates than transactional emails. Mixing these on a single domain and shared IP can jeopardize the deliverability of your most important messages. Understanding why your emails fail is key to avoiding these pitfalls.
How subdomains isolate reputation
Subdomains allow you to create distinct sender reputations, even if you're using a shared IP for the underlying sending infrastructure. Instead of sending all your emails from yourdomain.com, you can use promo.yourdomain.com for marketing and trans.yourdomain.com for transactional emails. This setup lets mailbox providers like Gmail and Outlook assign separate reputations to each stream.
The main benefit of this isolation is reputation protection. If your promotional campaigns experience deliverability issues, like high spam complaints, that bad reputation is primarily contained within the promo.yourdomain.com subdomain. Your critical transactional emails, sent from trans.yourdomain.com, are much less likely to be affected. This separation ensures that your most important communications maintain high inbox placement rates. To learn more about this, explore why separating transactional and marketing emails is vital.
Single domain on shared IP
All email types (promotional, transactional, corporate) share one domain and, consequently, one domain reputation. This means a single, unified identity for all your email sending, regardless of content.
Risk profile: High risk of promotional email issues (e.g., spam complaints, low engagement) negatively impacting transactional emails.
Deliverability: Deliverability for critical emails can fluctuate based on the performance of bulk sends.
Troubleshooting: Harder to pinpoint the cause of deliverability issues when all traffic is mixed.
Subdomains on shared IP
Different subdomains are used for different email types (e.g., promo.yourdomain.com, trans.yourdomain.com), allowing each to build a separate reputation.
Risk profile: Lower risk to transactional emails, as promotional issues are contained within their subdomain.
Deliverability: Improved stability for transactional emails, with higher inbox rates for critical communications.
Troubleshooting: Easier to diagnose and resolve issues by analyzing specific subdomain performance.
It's important to remember that using subdomains on a shared IP is a strategy for reputation management at the domain level, not the IP level. The IP reputation is still shared. However, mailbox providers weigh domain reputation heavily, and isolating it through subdomains offers significant protection for your brand.
Separating promotional and transactional emails
Transactional emails are expected, often anticipated, and generally have very high engagement rates because they deliver essential information like receipts, password resets, or shipping updates. Mailbox providers, therefore, prioritize their delivery.
Promotional emails, on the other hand, are often unsolicited and can generate lower engagement, higher unsubscribe rates, and more spam complaints. This is why separating these email types with subdomains is a deliverability best practice. By doing so, any negative signals from your promotional sends are less likely to bleed over and harm your transactional email performance.
Best practice: dedicated subdomains
For optimal deliverability and reputation management, it's highly recommended to use separate subdomains for your different email streams. This creates distinct sending reputations for each, protecting your most critical communications from issues affecting bulk mail. This approach aligns with broader recommendations to use different infrastructure for transactional and marketing emails.
Transactional: Use a subdomain like trans.yourdomain.com or mail.yourdomain.com.
Promotional: Use a different subdomain, such as marketing.yourdomain.com or news.yourdomain.com.
Corporate/Admin: You might even designate your root domain yourdomain.com for standard corporate communications.
This deliberate segmentation ensures that if your marketing efforts hit a snag, your essential transactional emails continue to flow without disruption. It's a fundamental step in building a resilient email program, especially on a shared IP.
Practical steps for subdomain implementation
Implementing subdomains primarily involves configuring your DNS records. Each subdomain will need its own SPF, DKIM, and DMARC records. Your ESP typically provides the specific records you need to add. Ensuring proper authentication (SPF, DKIM, DMARC) for each subdomain is critical for establishing its independent reputation.
Once configured, it’s vital to continually monitor the deliverability of each subdomain. Pay close attention to engagement metrics, complaint rates, and whether your emails are landing in the inbox or spam folder. Tools that provide blocklist monitoring and DMARC reports can help you identify and address issues promptly. Understanding what happens when your domain is blacklisted can provide valuable context for these metrics.
Email type
Recommended subdomain example
Key considerations
Transactional
trans.yourdomain.com
Highest priority, critical for user experience, high engagement.
Promotional
marketing.yourdomain.com
Lower engagement, higher risk of complaints, requires careful list management.
Corporate/Internal
mail.yourdomain.com
General business communications, internal emails, often uses the main domain for visibility.
Even on a shared IP, using subdomains for your different email types significantly enhances your ability to manage and protect your sender reputation. It's a proactive step that pays dividends in consistent deliverability for all your communications.
Views from the trenches
Best practices
Segment your email streams: Use distinct subdomains for promotional and transactional emails to isolate their reputations.
Prioritize transactional emails: Ensure your most critical messages have the best chance of inbox delivery.
Monitor each subdomain: Keep a close eye on individual deliverability metrics and sender reputation.
Common pitfalls
Ignoring subdomain performance: Assuming all subdomains will perform the same, leading to missed deliverability issues.
Neglecting authentication: Failing to properly configure SPF, DKIM, and DMARC for each new subdomain.
Over-reliance on shared IP: Not considering a dedicated IP for high-volume, critical transactional sending when volumes grow.
Expert tips
Start with a subdomain for promotional mail, especially if you anticipate deliverability challenges.
If promotional mail has high engagement, you might not need to segregate immediately, but it's a good long-term strategy.
Using separate subdomains for transactional, promotional, and even corporate email offers the highest level of reputation isolation.
Expert view
Steve589 from Email Geeks says using a subdomain for the DKIM signature is a good practice, especially if promotional emails are expected to cause problems.
2024-05-15 - Email Geeks
Marketer view
A marketer from Email Geeks says they are concerned about keeping promotional and transactional emails on the same subdomain due to past spam issues, highlighting the importance of separation.
2024-05-15 - Email Geeks
Maximising your email program's potential
While operating on a shared IP address, employing subdomains for your promotional and transactional email streams is a robust strategy. It enables you to segment your sending reputation, thereby protecting the deliverability of your critical transactional messages from the lower engagement rates or higher complaint volumes often associated with promotional campaigns.
This practice is about proactively managing risk and optimizing your email program's performance. By establishing separate domain reputations, even when sharing an IP, you build a more resilient infrastructure that can adapt to challenges without compromising your most vital communications. Continuous monitoring and adherence to email best practices for each subdomain remain paramount for sustained success.
Gmail and 
Outlook assign separate reputations to each stream.
