How to move sender reputation between ESPs?

Here's the short version: IP reputation doesn't transfer. Domain reputation does. The implications of that distinction shape everything about how you should approach a migration. Your domain reputation, built up through consistent sending with your own domain, travels with you because it's tied to the domain itself, not to the infrastructure that sends it. Mailbox providers like Google track reputation at the domain level. If you've been sending legitimate, well-engaged campaigns from newsletter@yourdomain.com for three years, that history follows you to a new ESP.

Your IP reputation is a different story. New IP addresses at your new ESP have no history. Even if you're sending the same content to the same list, mailbox providers don't know that yet. You'll need to warm those IPs up from scratch, gradually increasing volume while demonstrating engagement. Skip the warmup and you'll likely see temporary deliverability dips as providers decide whether to trust the new IPs.

Engagement history is also non-transferable. Your old ESP's internal records of who opened and clicked stay with your old ESP. But you can bring your most engaged subscribers, identified through your own analytics or exported engagement data, to the front of the migration queue. Start your warmup with those people. High engagement from your first sends on the new IPs helps build positive reputation faster.

Use your Google Postmaster Tools data during the migration to track your domain reputation score. If it holds steady or improves, you're in good shape. If it dips, something in the migration is triggering negative signals and you need to investigate before scaling up.