Can I use one subdomain for multiple email sending tools?

While technically possible for sending, sharing a subdomain across multiple email tools is generally not recommended due to limitations with MX records (which can only point to one server for mail reception) and the increased risk of one tool's poor sending reputation negatively impacting all others on that shared subdomain. It's better to separate by mailstream.

How do subdomains help with sender reputation?

Using separate subdomains helps isolate sender reputation. If one type of email (e.g., cold outreach) experiences deliverability issues or gets blocklisted, the negative impact is contained to that specific subdomain, protecting the reputation of your main domain and other critical email streams.

What is the SPF 10-lookup limit and how do subdomains help?

The SPF (Sender Policy Framework) record has a 10-lookup limit. When you use multiple email sending tools, each tool adds entries to your SPF record, often leading you to exceed this limit. Subdomains allow each tool to have its own SPF record on its dedicated subdomain, circumventing this limitation for your root domain.

Should I separate subdomains by email tool or by mailstream (type of email)?

It's best practice to separate mailstreams, not just tools. For example, use one subdomain for all marketing emails (regardless of the marketing automation tool) and another for all transactional emails. This provides better reputation isolation and management based on email type.

Best practices for using unique or shared email subdomains across multiple sending tools? - Technical - Email deliverability - Knowledge base

As organizations scale, they often adopt a diverse array of email sending tools for various functions, from marketing automation to transactional notifications. This multi-tool environment can quickly lead to complex DNS configurations and potential deliverability challenges. A common hurdle arises when managing Sender Policy Framework (SPF) records, particularly the 10-lookup limit imposed by the SPF specification, which can be easily exceeded when multiple tools require their own SPF entries.

This scenario often prompts the question: should you use a unique subdomain for each email sending tool, or can multiple tools share a single subdomain? The answer isn't always straightforward. It depends on a variety of factors, including your specific email sending volumes, the types of emails being sent, and your overall risk tolerance for sender reputation issues.

Navigating this choice requires a deep understanding of how subdomains impact email authentication, sender reputation, and overall deliverability. My aim is to shed light on these considerations, helping you formulate a robust subdomain strategy that ensures your emails consistently reach the inbox.

The role of subdomains in email authentication

Subdomains are essentially extensions of your primary domain (e.g., marketing.yourdomain.com or transactional.yourdomain.com). They act as independent entities for email sending, allowing you to isolate different types of email traffic. This isolation is crucial for managing sender reputation, as problems with one subdomain are less likely to impact the reputation of your main domain or other subdomains. For more on this, you can explore why to use email subdomains.

The primary technical reason to use subdomains, especially when dealing with multiple sending tools, revolves around email authentication protocols like SPF and DKIM. SPF records specify which mail servers are authorized to send email on behalf of a domain. Critically, an SPF record can only contain up to 10 DNS lookups. Exceeding this limit can lead to SPF authentication failures, causing your emails to be flagged as spam or rejected. Subdomains allow each tool to have its own dedicated SPF record, thereby avoiding the 10-lookup limit on your root domain.

DKIM, or DomainKeys Identified Mail, provides an additional layer of authentication by adding a digital signature to your outgoing emails. Each sending tool or platform will typically provide its own DKIM keys, which are published as CNAME records or TXT records on your domain (or subdomain). When using subdomains, you can configure distinct DKIM records for each, ensuring proper authentication for all your email streams. Understanding these foundational elements is key for DNS lookups and subdomain usage.

Unique versus shared subdomains

The core decision is whether to assign a unique subdomain to each email sending tool or to consolidate multiple tools under a shared subdomain. While it's technically possible for one subdomain to send mail from multiple tools (provided the SPF records cover all sending IPs and DKIM is properly configured), it's generally not recommended. A shared subdomain can create complications, particularly concerning mail reception and reputation management. You can't usually receive mail to a single subdomain with multiple tools simultaneously, as MX records can only point to one destination.

A more effective strategy often involves separating subdomains by mailstream rather than strictly by tool. This means you would group similar types of email traffic (e.g., marketing, transactional, cold outreach) onto their own dedicated subdomains, regardless of which specific tool sends them. For instance, all transactional emails, whether from your CRM or an e-commerce platform, could use tx.yourdomain.com. This approach allows for better reputation isolation and management.

The Mailgun and M3AAWG organizations, for instance, highlight the importance of separating different email types. They suggest using distinct subdomains for marketing emails versus transactional emails as a best practice. This separation helps to protect the deliverability of critical transactional messages from the lower reputation risks often associated with marketing campaigns. Consider the various functions your email sending tools fulfill when designing your subdomain architecture.

Unique subdomain per tool/mailstream

Reputation isolation: Issues with one mail stream, like cold outreach, are contained to its specific subdomain, protecting your core domain and other email activities from being blacklisted (or blocklisted).
Easier diagnostics: If you encounter deliverability problems or land on a blocklist, you can quickly identify the source by looking at which subdomain is affected.
Simplified DNS records: Each subdomain can have its own SPF and DKIM records, preventing the common SPF 10-lookup limit issue on your root domain.

Shared subdomain across multiple tools

Reputation risk: Poor performance or blocklisting on one tool can negatively affect the reputation of all other tools sharing that subdomain.
Complex DNS: Combining SPF records for multiple tools on a single subdomain can quickly hit the 10-lookup limit, leading to authentication failures. DKIM selectors might also collide.
Limited mail reception: MX records for a subdomain can typically only point to one mail server, limiting the ability of multiple tools to handle incoming mail for that subdomain.

Reputation management and deliverability

Sender reputation is paramount for email deliverability. Every email you send contributes to the reputation of your sending domain and, by extension, your root domain. When a subdomain's reputation declines, perhaps due to high bounce rates, spam complaints, or being listed on a blocklist (or blacklist), it can impact the overall perception of your primary domain. This is why reputation isolation through dedicated subdomains for different mailstreams is a critical best practice. Learn more about understanding your email domain reputation.

For instance, if your cold outreach subdomain experiences high spam complaint rates, it's far better for that negative impact to be contained to outreach.yourdomain.com than to affect your transactional subdomain tx.yourdomain.com. This separation allows you to manage the reputation of each mail stream independently and take targeted corrective actions if needed. It also makes it easier to comply with evolving sender requirements, such as those from Outlook's new sender requirements.

While subdomains offer significant advantages for reputation management, it's important to remember that a severe blocklisting of a subdomain can still have broader implications. Internet Service Providers (ISPs) and blocklist operators often link subdomains to their root domains. Therefore, maintaining good sending hygiene across all your subdomains is crucial to protect your entire domain's sending capabilities. Check out our in-depth guide to email blocklists for more information.

Reputation management best practices

Isolate high-risk sending: Assign cold outreach or other potentially risky email types to their own unique subdomains. This prevents issues from impacting your main sending reputation.
Monitor subdomain reputation: Regularly check the deliverability and sender reputation of each subdomain using tools like Google Postmaster Tools (for Gmail) or other monitoring services.
Maintain consistent sending practices: Even with separate subdomains, ensure all your sending adheres to best practices, including list hygiene, content quality, and proper authentication (SPF, DKIM, DMARC).
Warm up new subdomains: When introducing a new subdomain, gradually increase sending volume to build a positive reputation with ISPs.

Technical considerations and setup

When configuring subdomains for multiple sending tools, you'll primarily deal with DNS records. Your email service providers (ESPs) or marketing automation platforms will typically provide specific TXT records for SPF and CNAME records for DKIM. It's crucial that these records are correctly set up for each subdomain you intend to use. For tools that support CNAME authentication, it can sometimes simplify DNS management by allowing the ESP to manage the SPF and DKIM records on their end, pointed to by your CNAME.

However, the primary concern with using the same subdomain for multiple platforms lies in the MX record. An MX record (Mail Exchange record) specifies the mail server responsible for accepting email messages on behalf of a domain name. If multiple tools require the MX record for a subdomain to point to their servers for bounce processing or replies, you'll encounter a conflict, as an MX record can only point to one destination. This often makes sharing a subdomain across tools impractical if reply mail handling is critical for all of them.

Beyond SPF and DKIM, implementing DMARC (Domain-based Message Authentication, Reporting & Conformance) is essential for monitoring and enforcing your email authentication policies. DMARC works in conjunction with SPF and DKIM, providing reporting that helps you understand email flows and identify unauthorized sending. You should implement a DMARC policy for your root domain and all active subdomains. For more detailed technical guidance on SPF and subdomains, consider consulting resources like the SPF for subdomain guide.

Example SPF record for a subdomain used by Google Workspace and SendGridTXT

v=spf1 include:_spf.google.com include:sendgrid.net ~all

Views from the trenches

Best practices

Always separate your transactional emails from marketing or bulk campaigns using distinct subdomains.

Use clear and descriptive subdomain names (e.g., 'updates' for transactional, 'promo' for marketing).

Implement DMARC for all subdomains, starting with a 'p=none' policy to gather reports.

Regularly monitor your subdomain reputation and blocklist status to catch issues early.

Common pitfalls

Hitting the SPF 10-lookup limit by attempting to consolidate too many sending tools on a single record.

Using your primary root domain for high-volume or cold outreach email sending, risking its overall reputation.

Ignoring DMARC reports, missing critical insights into email authentication failures and potential abuse.

Not warming up new subdomains or sending too much volume too quickly, leading to poor deliverability.

Expert tips

Consider CNAME authentication when offered by your sending tool; it can simplify DNS management.

Prioritize separating mailstreams (e.g., transactional, marketing, cold outreach) over separating by tool.

Understand that even with subdomains, a severe blocklist on one can sometimes affect the root domain's perception.

If your security team cites 'too many DNS records,' clarify if they mean SPF lookup limits or other authentication types.

Marketer view

Marketer from Email Geeks says there isn't a technical barrier to one subdomain sending from multiple tools, but receiving mail to a subdomain with multiple tools simultaneously isn't possible.

2019-11-05 - Email Geeks

Marketer view

Marketer from Email Geeks says it's possible to use one subdomain across multiple tools, provided SPF records permit all tool IP addresses and DKIM is properly configured.

2019-11-05 - Email Geeks

Strategic subdomain management

The decision to use unique or shared email subdomains across multiple sending tools largely hinges on balancing technical feasibility with reputation management. While sharing a subdomain might seem simpler initially, the complexities of MX records, SPF lookup limits, and the magnified risk of reputation damage often make a unique subdomain strategy the more robust and reliable choice.

By strategically segmenting your email traffic across dedicated subdomains, you can enhance deliverability, simplify troubleshooting, and protect your primary domain's reputation from the specific challenges of different email streams. This proactive approach to subdomain management is a cornerstone of effective email security and deliverability.