What are the best practices for migrating ESPs with subdomain and IP warmups?

Key findings

• IP warmup is essential: Even if you keep your existing sending subdomain, switching to new IP addresses necessitates a full warmup process. ISPs will perceive these IPs as new, and a gradual increase in sending volume is crucial to build a positive reputation with them. You can find more information on best practices for dedicated IP warmup.
• Subdomain consistency: While the exact FQDN (fully qualified domain name) of your sending subdomain is less critical than the From domain, preserving it can offer some continuity. However, if there's any overlap in sending from both ESPs, you must set up a new subdomain to avoid DNS conflicts and ensure proper authentication.
• Domain reputation's role: After the initial SMTP transaction, your domain's reputation heavily influences inbox placement. While the IP gets your email accepted, the domain often determines where it lands within the recipient's inbox (e.g., primary, promotions, or spam).
• Separating mail streams: It is a standard best practice to use separate sending domains (or subdomains) and potentially separate IPs for marketing emails versus transactional emails. This segregation helps protect your transactional email deliverability from potential issues with marketing campaigns.

Key considerations

• DNS configuration: Ensure that your DNS records (SPF, DKIM, DMARC) are correctly configured for your new ESP and sending domains. Incorrect setup can lead to severe deliverability problems. You can use our free DMARC record generator to assist with this.
• Phased migration: Implement a phased migration strategy where you gradually shift sending volume from your old ESP to the new one. This allows for parallel sending and controlled warmup. This approach is elaborated upon by EmailLabs, recommending a gradual shift of traffic starting with a small percentage.
• Volume distribution: Consider your typical sending patterns. If you have large, infrequent sends, you might need more dedicated IPs or a longer warmup period than if your volume is consistently spread throughout the week.
• Sender reputation transfer: Understand that existing sender reputation from your old IPs does not directly transfer to new ones. A new IP must build its own reputation from scratch. This is a crucial aspect to manage to avoid emails going to spam.

Key opinions

• New IP/domain warming is mandatory: Marketers universally agree that any new IP address or a new IP/subdomain combination will require a dedicated warmup process, regardless of past domain performance. Mailbox providers like Google, Microsoft, and Yahoo will treat these as new sending entities. Kickbox emphasizes that creating new subdomains whenever possible is recommended when switching ESPs.
• Subdomain overlap avoidance: A key concern for marketers is the impossibility of having a single subdomain (e.g., em2468.clientdomain.tld) pointing to two different ESP infrastructures simultaneously. This necessitates a new subdomain if parallel sending (using both old and new ESPs) is part of the migration strategy.
• Dedicated IP volume capacity: For significant volumes (e.g., 7-10 million marketing emails per month), a single dedicated IP can be sufficient if the sender maintains a strong reputation and distributes sends appropriately, though adding more IPs later is an option if throttling occurs. It depends on the specific ESP and the receiving ISPs' policies.
• Strategic sending separation: Splitting marketing and transactional emails to different sending domains and IPs is widely recommended, even when originating from the same system. This isolates the reputation of critical transactional mail from higher-risk marketing campaigns.

Key considerations

• Phased warmup approach: Marketers often plan to warm up new IPs/subdomains by sending to their most engaged recipients on the new platform, while less engaged segments continue to receive emails from the old ESP. This allows for a smooth transition and builds early positive signals.
• DNS management during overlap: During any period of parallel sending, careful DNS planning is vital to ensure that each ESP's sending infrastructure is correctly configured and authenticated without conflicting CNAME records or other DNS entries for the same subdomain. Learn more about consolidating email sending platforms.
• Reputation tracking: It is important for marketers to closely monitor sender reputation metrics through tools like Google Postmaster Tools and their ESP's analytics during the warmup period to identify and address any deliverability issues quickly. This can help prevent the new IP from getting on a blocklist or blacklist.
• Domain vs. IP perception: Marketers should understand that while IP reputation is crucial for initial acceptance, a strong domain reputation built over time is what primarily dictates inbox placement and long-term deliverability. Onesignal's blog points out that domain warm-up is always necessary when migrating to a new platform.

Key opinions

• Domain's primary role in delivery: Experts often clarify that while the IP gets your email past the SMTP transaction, it is primarily the domain's reputation that determines successful inbox placement at many consumer-facing domains. This is a crucial nuance for senders to grasp during migration. This relationship is often detailed in guides like Email Deliverability Issues: Getting Your Messages to the Inbox.
• Hostname importance: The reverse DNS of the IP and the hostname used in the HELO command are generally less significant than the 5321.From domain, provided they are syntactically valid and do not resemble suspicious or compromised sources. They act as base identifiers, but domain reputation is paramount.
• No stealing hostnames: A critical warning from experts is to avoid configuring the new ESP to use hostnames that are still associated with or actively used by the old ESP. This can lead to severe deliverability problems and should be prevented at all costs.
• Warmup universality: Whether you choose to keep or change your subdomain, a new IP/domain combination will always necessitate a complete warmup process to establish trust with mailbox providers and ensure optimal inbox placement.

Key considerations

• Smooth cut-over and authentication: Experts stress that the primary focus during migration should be on executing a smooth cut-over and ensuring that all email authentication records (SPF, DKIM, DMARC) are updated and correctly configured for the new sending infrastructure. This foundation is key to maintaining trust. See best practices for retaining sender reputation during migration.
• Customizing hostnames: While an ESP's default hostnames are usually acceptable, customizing them to match your brand (e.g., using mail.yourdomain.com) is a valid option. This can help with brand recognition and consistency, though it doesn't bypass the need for warmup.
• Transactional vs. marketing split: The recommendation to separate marketing and transactional email streams is standard across the industry. This strategy protects the high deliverability of essential transactional emails from reputation risks associated with promotional sends. A comprehensive guide on this can be found at AWS Blogs.
• IP warming strategies by ISP: Different ISPs (like Google, Microsoft, Yahoo) may have slightly different thresholds and speeds for IP warmup. Tailoring your warmup strategy to cater to the specific requirements of each major ISP can optimize deliverability across your recipient base. You can explore IP and domain warm-up strategies for Gmail and Microsoft.

Key findings

• Domain vs. IP warmup necessity: Documentation consistently states that domain warm-up is always necessary when migrating to a new platform to establish its reputation. IP warm-up, however, is specifically required when acquiring dedicated IP addresses, as shared IPs often leverage an existing, warmed reputation. OneSignal's blog confirms that domain warm-up is always necessary.
• Gradual traffic shifting: A best practice for warming involves gradually shifting email traffic from the old infrastructure to the new. This means starting with a small percentage of your sending volume (e.g., 5%) on the new setup and slowly increasing it over time. EmailLabs recommends gradually shifting the traffic over time.
• Content for warming: Documentation advises using your best-performing campaigns with high user engagement and low complaint/unsubscribe rates for the initial warm-up sends. These positive signals help build a strong reputation quickly. MessageGears specifically recommends using a campaign with high user engagement.
• Time allocation for warmup: It is crucial to allocate sufficient time for warming and testing each new IP before the official changeover. This ensures the IP is fully warmed and capable of handling desired volumes without deliverability issues, as emphasized by Ongage.

Key considerations

• DNS records: Ensure that the branded sending subdomain is not already in use within your DNS before configuring it for the new ESP. Proper DNS setup, including CNAME records, is fundamental for email authentication and routing. Klaviyo's help center highlights the importance of ensuring the subdomain is not already being used.
• Automated vs. manual warmup: Some ESPs offer automated IP warmup services, which can streamline the process by managing throttling automatically. For those without such features, a manual warmup process following established best practices is necessary. This is discussed in Email on Acid's deliverability guide.
• Monitoring during warmup: Continual monitoring of deliverability metrics, such as inbox placement, open rates, and complaint rates, is crucial during the warmup phase. This allows for quick adjustments if any issues arise and helps confirm that the new IPs and domains are building positive reputations. Consider blocklist monitoring to catch potential issues.
• ISP-specific guidelines: Some mailbox providers publish specific guidelines for IP and domain warming. Adhering to these recommendations can significantly improve the success rate of your migration. For example, AWS offers a comprehensive guide to IP and domain warming for migrating to Amazon SES.