What checklist of things to check when switching from sending emails from a primary domain to a subdomain?

Key findings

• Warm-up period: Even if only changing the domain and not the IP or sending infrastructure, a warm-up period is necessary for the new subdomain. This allows mailbox providers' machine learning filters to recognize the new domain as legitimate and prevent it from being flagged as suspicious. For more on this, read when switching to a new subdomain.
• DNS records: Beyond SPF, DKIM, and DMARC, ensure A records, MX records, and any appropriate redirects are correctly configured for the subdomain. This confirms that the subdomain is properly linked to your sending infrastructure.
• Separate email streams: For high-volume senders, it is often beneficial to use distinct subdomains for different email types (e.g., promotional versus transactional). This isolates reputation, protecting critical transactional emails from potential issues with marketing campaigns.
• Authentication checks: After setting up, send test emails and verify that SPF, DKIM, and DMARC all pass. This immediate check provides a quick indicator of your setup's health and can be done easily via a Gmail original message view.

Key considerations

• Recipient aggregation: Be aware that some email receivers aggregate reputation up to the top-level domain (TLD), while others treat subdomains as independent entities. This variability influences the impact of your subdomain reputation on your overall domain. Learn more about email subdomains in Mailgun's guide to the basics.
• Dual DKIM signing: Consider adding a second DKIM signature from the new subdomain alongside your existing primary domain DKIM. This dual DKIM strategy helps gradually introduce the new subdomain's reputation to machine learning filters.
• Header order: When using dual DKIM signatures, the order in which they appear in the email header matters. Mailbox providers typically prioritize the signature closer to the top of the header for reputational assessment. Aim for the new signature to eventually be at the top after a warm-up period.
• Maintenance overhead: While separating email streams offers deliverability benefits, it also increases the management overhead. Assess if the volume and risk profile of your email streams justify the additional work.
• Monitoring: Continuously monitor your new subdomain's deliverability and reputation metrics using tools like Google Postmaster Tools. This helps you quickly identify and address any issues that arise during or after the transition.

Key opinions

• Warm-up is essential: Most marketers agree that even when only changing from a primary domain to a subdomain, a warm-up period is required. This helps mailbox providers trust the new sending identity.
• Separate subdomains for stream types: It is generally recommended to use separate subdomains for different email streams, such as promotional emails and transactional emails. This segregation protects the reputation of critical communications.
• Benefit outweighs work for high volumes: For significant email volumes, the deliverability benefits of separating streams via subdomains usually justify the extra setup and maintenance work.
• DNS checks are critical: Beyond SPF, DKIM, and DMARC, marketers highlight the need to check A records, MX records, and other redirects to ensure proper configuration.

Key considerations

• Initial setup verification: Marketers frequently advise sending test messages to personal inboxes (like Gmail) and inspecting the original message headers to confirm SPF, DKIM, and DMARC pass statuses.
• Future-proofing: Even without immediate deliverability concerns, many marketers choose to move to subdomains to future-proof their email programs against potential reputation issues down the line.
• Complexity vs. benefit: While useful, managing multiple subdomains can add complexity. Marketers suggest evaluating if the volume and nature of your emails truly necessitate this separation. For details on sharing subdomains, see best practices for using subdomains.
• Secondary domain verification: Some marketers try connecting a new subdomain as a secondary domain for verification before making it primary.

Key opinions

• Warm-up is crucial: Experts emphasize that a warm-up period, even a week long, is necessary to allow machine learning filters to recognize the new subdomain as legitimate and not compromised.
• Strategic DKIM signing: A recommended strategy is to add a second DKIM signature for the new domain alongside the existing one. This gradually introduces the new domain's signature to mailbox providers' filters. For more on DKIM, check our guide to DMARC, SPF, and DKIM.
• Header order matters: The order of DKIM signatures in the email header impacts which signature ISPs prioritize for reputation. The final signature (closer to the top of the header) is generally given reputational primacy.
• Volume dictates separation: While separating promotional and transactional emails onto different subdomains is a good general practice, experts advise considering the actual email volumes. Very high transactional volumes might not see significant benefit from separation.

Key considerations

• DKIM independence: The DKIM specification does not require the d= tag in the signature to be related to any other domain in the message. It merely indicates the domain taking responsibility for the message. More on this at Word to the Wise.
• DMARC alignment: DMARC is the protocol that introduces the concept of alignment, requiring only one authentication string to align and pass. This makes the dual DKIM strategy effective, as the primary, known signature can still pass DMARC while the new one warms up.
• Influencing signature order: The signing order, which dictates the header order, can be MTA-specific. It's recommended to consult with your Email Service Provider (ESP) or MTA configuration to ensure the desired order of signatures is achieved. Learn how to configure SPF for subdomains in our guide to SPF configuration.
• Risk mitigation: Separating high-volume promotional mail from triggered mail protects essential communications. If promotional mail faces deliverability issues, the triggered mail's reputation on its separate subdomain remains intact.

Key findings

• Subdomain benefits: Documentation frequently states that using subdomains for email sends is an effective way to boost deliverability and reputation, as each subdomain can develop its own independent reputation.
• DNS configuration: Setting up SPF, DKIM, and DMARC records for the new subdomain is a critical step, ensuring proper email authentication and compliance.
• Testing and verification: After configuring DNS and email server settings, it is essential to test and verify that emails sent to and from the subdomain are delivered correctly and authenticate properly.
• Reputation isolation: An email subdomain functions as a separate entity within the root domain, allowing it to have its own reputation rate without negatively impacting other subdomains or the main domain.

Key considerations

• Sender address formatting: Ensure that the email address used for sending (e.g., user@sub.domain.com) is correctly configured in your mail sending application.
• Domain authorization status: Regularly check your subdomain's authorization status within your ESP's settings, typically under DNS Records & Settings, to ensure continuous validation.
• Comprehensive setup guide: Follow a detailed setup guide that includes steps like creating an account, adding your domain, configuring DNS records, and connecting your domain to the sending platform.
• Best practices for deliverability: Adhere to general email best practices and consider email warm-up even for subdomains, as this is crucial for long-term deliverability.