Primary Domain vs. Subdomain Selection Guide

Analysis of Key Decision Factors

Based on research into best practices from multiple email service providers, here's a detailed analysis for choosing between a primary domain and subdomain:

🔴 Primary Domain (APEX) Usage Scenarios

Suitable Use Cases:

Startups or new projects that haven't established an email system yet
Used exclusively for transactional and notification emails, not mass marketing
Strong brand consistency is required with a preference for simple sender addresses
Low email volume with manageable risk

Risks and Limitations:

DNS Conflict Risk: If the primary domain is already used for corporate email (such as Google Workspace or Office 365), configuring MX records will cause existing email outages
Reputation Sharing Risk: All email activity shares the same domain reputation, and a single issue can affect the entire domain
Limited Flexibility: Unable to isolate and manage different types of email separately

🟢 Recommended Subdomain Solutions

Advantages:

Risk Isolation: Subdomains have independent email reputations from the primary domain
Business Segmentation: Subdomains can be created for specific use cases:
- marketing.company.com → Marketing emails
- transactions.company.com → Transaction notifications
- alerts.company.com → System alerts
Flexible Configuration: Does not impact existing primary domain email services
Test-Friendly: New projects can be tested without impacting the primary brand

Industry Best Practices:

Primary Domain: company.com → Internal corporate communications
Subdomain: send.company.com → Email delivery
Subdomain: news.company.com → Newsletters

Decision Flowchart

Decision Path 1: Existing Email System Check

Is the primary domain currently configured with MX records?

Yes → Must use a subdomain
No → Proceed to next decision point

Decision Path 2: Email Type Analysis

What types of emails are primarily sent?

Transactional Emails (registrations, orders) → Consider primary domain
Marketing Emails (promotions, newsletters) → Recommend subdomains
Mixed Types → Strongly recommend subdomains

Decision Path 3: Sending Volume Assessment

What is the expected average daily send volume?

< 10,000 emails → Either primary domain or subdomain is acceptable
10,000-100,000 emails → Subdomains recommended
> 100,000 emails → Subdomains required

Technical Implementation Recommendations

Subdomain Naming Conventions

Recommended formats:

send.[primary domain] - General sending subdomain
news.[primary domain] - Newsletter-specific
notify.[primary domain] - System notifications only

Avoid these names:

mail.[primary domain] - May conflict with corporate email
smtp.[primary domain] - Has strong technical connotations
admin.[primary domain] - Higher security risk

Hybrid Strategy Recommendations

For mature enterprises, a tiered strategy is recommended:

Primary domain: company.com

Reserved for corporate email (Google Workspace/Office 365)

Subdomains:

send.company.com → Email sending service
news.company.com → Marketing emails only
alerts.company.com → System monitoring alerts

Real Case Studies

Case 1: E-commerce Platform

Primary Domain: store.com → Corporate email and official website
Subdomain: mail.store.com → Order confirmations and shipping notifications
Subdomain: promo.store.com → Promotional emails

Result: Increased complaint rates for marketing emails do not affect transactional email delivery

Case 2: SaaS Service Provider

Primary Domain: service.com → Reserved for corporate communications
Subdomain: notify.service.com → Product notification emails
Subdomain: news.service.com → Product updates and news

Result: Clear email categorization allows users to subscribe selectively

Summary and Recommendations

Prefer Subdomains When:

Company has an established email system
Sending marketing or bulk emails
Need to test and optimize sending strategies
Focusing on email reputation management

Consider Primary Domains When:

New project with no legacy systems
Sending only critical transactional emails
Pursuing minimalistic brand presentation
Small and controllable sending volume

Best Practice

Regardless of your choice, it's recommended to start with a subdomain and evaluate whether to use the primary domain based on actual needs after your sending reputation stabilizes.