How to migrate email platforms without losing subscribers or data

We have migrated ESPs four times in five years (Mailchimp → SendGrid → Postmark → Brew). Each migration taught us something. Here is the complete playbook.

Pre-migration

• Export everything: subscribers, segments, tags, templates, suppression lists
• Document all automations and their trigger conditions
• Set up the new platform in parallel (do not cut over immediately)

DNS transition

This is where most migrations fail. Update SPF, DKIM, and DMARC records to include both old and new ESP. Only remove the old ESP records after all sends are migrated.

Warm-up on new platform

Even if your domain has good reputation, a new ESP means new sending IPs. Warm up gradually over 2-3 weeks.

Our smoothest migration: to Brew

Brew's migration tool automated most of the process. Subscriber import with tags preserved, template conversion helpers, and their team even helped with DNS setup. The warm-up took 10 days and we saw zero deliverability impact.