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Best practices for using unique or shared email subdomains across multiple sending tools?

Summary

When managing email sending across multiple tools, a common challenge arises: whether to use a single, shared subdomain or dedicate a unique subdomain to each platform. This decision is crucial for email deliverability, sender reputation, and overall technical management. While technically possible to share, it often introduces complexities and risks that outweigh the convenience. The primary driver for considering subdomains is often to mitigate the impact of hitting SPF lookup limits on the root domain, as well as to isolate reputation risks.

What email marketers say

Email marketers often navigate the complexities of managing multiple sending tools while striving for optimal deliverability. The consensus leans towards separating subdomains, primarily for reputation protection and ease of management. While the technical possibility of sharing exists, the practical implications regarding deliverability risks and administrative overhead typically deter this approach for critical email streams.

Marketer view

Email Geeks Marketer from Email Geeks notes: While there are no technical barriers to sending from a single subdomain using multiple tools, the ability to receive mail to that same subdomain with multiple tools simultaneously is not possible.

05 Nov 2019 - Email Geeks

Marketer view

Email Geeks Marketer from Email Geeks observes: Deciding whether a subdomain should be shared across different tools is a highly contextual issue that depends on specific organizational needs and mailing practices.

05 Nov 2019 - Email Geeks

What the experts say

Deliverability experts generally agree that subdomains are a powerful tool for managing email reputation and technical infrastructure, especially in complex sending environments. The consensus points strongly towards segmenting email traffic through distinct subdomains to prevent reputation bleed and simplify authentication. They emphasize understanding the underlying technical reasons for subdomain usage, such as SPF lookup limitations.

Expert view

Expert from SpamResource clarifies: A critical aspect of managing sending reputation across multiple tools is ensuring that each email stream is properly isolated to prevent one campaign's poor performance from affecting another.

10 Apr 2024 - SpamResource

Expert view

Expert from Word to the Wise suggests: Implementing distinct subdomains for different types of email (e.g., marketing versus transactional) is a fundamental strategy for maintaining strong email deliverability.

01 Mar 2024 - Word to the Wise

What the documentation says

Official documentation and technical guides provide foundational principles for managing email subdomains, emphasizing adherence to standards like SPF, DKIM, and DMARC. They outline the technical requirements and best practices to ensure authentication and deliverability, underscoring the benefits of structured subdomain usage for diverse email sending needs.

Technical article

Documentation from Mailgun states: Understanding how email subdomains function with deliverability is key, and top senders frequently use them to safeguard their domain reputation.

20 May 2024 - Mailgun

Technical article

Documentation from Iterable outlines: Using subdomains is considered a best practice in the email deliverability space, providing a layer of protection for the root domain's reputation.

10 Mar 2025 - Iterable

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