Summary
The consensus is that the optimal number of subdomains per dedicated IP for email sending hinges on email stream and volume. A key strategy involves isolating different email types (transactional, marketing, bulk) on separate subdomains to protect sender reputation. High email volume generally supports multiple subdomains, while low volume may benefit from a single subdomain to avoid diluting sender reputation. Using subdomains also facilitates better management of SPF and DKIM records, enhancing deliverability and security. Warming up new IPs is crucial for a smooth transition, and considering the specific needs of the email program is paramount.
Key findings
- Stream Isolation: Using separate subdomains for different email streams (transactional, marketing, etc.) isolates sender reputation, preventing deliverability issues in one stream from affecting others.
- Volume Dependence: High email volume can support multiple subdomains, while low volume may benefit from a single subdomain to maintain a concentrated reputation.
- SPF/DKIM Management: Subdomains enable more granular management of SPF and DKIM records, enhancing deliverability and security.
- IP Warmup: Warming up new IPs with gradual increases in volume is crucial for a smooth transition and maintaining sender reputation.
Key considerations
- Email Program Needs: Assess the specific needs of the email program to determine the optimal subdomain strategy.
- Volume Thresholds: Consider email volume when deciding on the number of subdomains; low volume may not justify multiple subdomains.
- SPF/DKIM Configuration: Ensure proper configuration of SPF and DKIM records for each subdomain to maximize deliverability and security.
- Reputation Monitoring: Monitor sender reputation for each subdomain to promptly address any deliverability issues.
- Transition Planning: Plan for a gradual IP warmup when transitioning to new dedicated IPs to avoid negatively impacting sender reputation.
What email marketers say
8 marketer opinions
The number of subdomains to use per dedicated IP address for email sending depends on several factors, primarily the volume and type of email being sent. Isolating email streams (transactional, marketing, etc.) onto separate subdomains is a common practice to protect sender reputation. This prevents issues in one stream from impacting the deliverability of others. High email volume can generally withstand multiple subdomains. However, very low email volume may benefit from a single subdomain to avoid spreading traffic too thinly, which can negatively impact deliverability. The overall goal is to manage and isolate sender reputation effectively, ensuring optimal email deliverability.
Key opinions
- Reputation Isolation: Using separate subdomains for different email types (transactional, marketing) isolates sender reputation, preventing one type of email from negatively affecting another.
- Volume Impact: High email volume is better suited for multiple subdomains, while very low volume may benefit from a single subdomain to avoid spreading traffic too thinly.
- Dedicated IP Threshold: A volume of approximately 1.5 million emails per month can generally justify the use of a dedicated IP address.
- Sender reputation: Using a sending subdomain helps increase deliverability, as sender reputation is tied to the sending subdomain rather than the main domain.
Key considerations
- Email Volume: Assess the volume of emails for each type of email stream. Ensure sufficient volume per subdomain to maintain a positive sender reputation.
- Email Type: Determine the types of emails being sent (transactional, marketing, etc.) and whether isolating them onto separate subdomains is necessary for reputation management.
- Dedicated IP Setup: If using multiple dedicated IPs, carefully monitor each email stream per IP to ensure consistent deliverability.
- Reputation Protection: Implement subdomains to protect the main domain’s reputation, ensuring that deliverability issues on one subdomain do not impact the primary domain's email.
Marketer view
Email marketer from Mailjet shares that using subdomains for different types of email (transactional, marketing, etc.) helps to isolate reputation and prevent one type of email from negatively impacting the deliverability of another.
18 Jan 2022 - Mailjet
Marketer view
Email marketer from Email Marketing Forum explains that if you have very low volume, consider using a single subdomain. Spreading volume across multiple subdomains with minimal traffic can negatively impact deliverability.
26 Dec 2023 - Email Marketing Forum
What the experts say
4 expert opinions
The advice from experts emphasizes the importance of subdomains for email deliverability and sender reputation management. The general recommendation is to use a separate subdomain for each distinct email stream (e.g., marketing, transactional, bulk). This isolation ensures that deliverability issues affecting one type of email do not impact others. Warming up new IPs is also crucial to ensure a smooth transition. Using subdomains for DKIM identifiers gives greater control. The specific needs of the email program dictates how best to use subdomains.
Key opinions
- Stream Isolation: Separate subdomains are advised for each distinct email stream (marketing, transactional, etc.) to isolate deliverability issues.
- DKIM Identifiers: Subdomains can be used for DKIM identifiers, providing more control over email authentication.
- IP Warmup: Warming up new IPs with small email volumes during the transition process is essential for a smooth deliverability.
- Deliverability: Subdomains are important for the optimization of deliverability.
Key considerations
- Email Program Needs: The optimal subdomain strategy depends on the specific email program and its objectives.
- DKIM Setup: Consider using subdomains for DKIM identifiers to enhance email security and authentication.
- Gradual Transition: Implement a gradual transition by warming up new IPs, to avoid damaging sender reputation during migration.
- Reputation Protection: Utilize subdomains to protect the primary domains reputation, maintaining optimal deliverability.
Expert view
Expert from Email Geeks shares to warm up when moving to ensure a smooth transition. Very small amounts from the new IPs and gradually reduce volume from the shared until you’ve moved everything.
13 Jul 2021 - Email Geeks
Expert view
Expert from Email Geeks explains that a subdomain should be used for each distinct mail stream, irrespective of the number of dedicated IPs. He also mentions it depends if you're using subdomains for DKIM identifiers.
19 Apr 2024 - Email Geeks
What the documentation says
4 technical articles
Technical documentation consistently emphasizes the benefits of using subdomains for email sending. Subdomains facilitate better organization of email infrastructure, enable more granular SPF records, and improve overall deliverability. They allow for easier management of DNS records and enhance security through separate DKIM keys for each subdomain. Aligning subdomains with specific use cases is recommended to isolate sender reputation and optimize email traffic.
Key findings
- Granular SPF: Subdomains enable more granular SPF records, which enhance deliverability.
- DNS Management: Delegating subdomains simplifies DNS record management and segments email traffic.
- Reputation Isolation: Multiple subdomains for different email types improves deliverability by isolating sender reputation.
- DKIM Management: Using subdomains makes it easier to manage DKIM keys, enhancing security and control.
Key considerations
- Infrastructure Organization: Plan your email sending infrastructure to effectively utilize subdomains for better organization.
- SPF Configuration: Configure SPF records for each subdomain to improve email authentication and deliverability.
- DNS Delegation: Consider delegating subdomains to simplify DNS management and segment email traffic.
- DKIM Key Management: Implement DKIM keys for each subdomain to enhance email security and sender authentication.
- Specific Use Cases: Align subdomains with specific use cases to better manage sender reputation and optimize deliverability.
Technical article
Documentation from RFC explains that using subdomains makes it easier to manage DKIM keys. Every subdomain can have its own DKIM key, which allows greater control and better security.
10 Jun 2025 - ietf.org
Technical article
Documentation from AWS explains that using multiple subdomains to differentiate email types improves deliverability by isolating sender reputation. They recommend aligning subdomains with specific use cases.
5 Jul 2021 - AWS
Can I use two subdomains on one dedicated IP address for different email types?
Can one dedicated IP be mapped to multiple sub-domains for email?
How many emails can I send per dedicated IP address before needing another one?
How many IP addresses should be used for an email marketing campaign of 50k emails?
How many subdomains should I create for my email sending, and what naming conventions should I use?
Should I use subdomains for outbound email delivery to improve deliverability?
