Migrating ESPs Without Losing Deliverability

The most common migration failure: Brand signs up for a new ESP on Monday, imports their full list on Tuesday, sends a campaign to 100K people on Wednesday. Inbox placement drops to 40%. Revenue from email craters.

Your new ESP has a new IP and sending domain. Inbox providers have no history on them. A sudden blast of 100K emails from an unknown sender looks like spam. That is the reputation you start with.

Export your full subscriber list with engagement data (last open date, last click date, purchase history, signup source). You need this data in the new ESP to maintain your segments.

Document all active automation flows, campaign templates, and segments in your current ESP. Screenshot the flow logic. Export the email HTML.

Run your entire list through an email verification service. Clean out invalids, spam traps, and role addresses before importing into the new ESP. Do not migrate dirty data.

Set up DNS authentication (SPF, DKIM, DMARC) for the new ESP before sending anything. Verify records propagate correctly.

Week 1: Send to your 500 most engaged subscribers only (clicked in last 14 days, purchased in last 30 days). These people will open and click, building positive reputation signals.

Week 2: Expand to your top 5,000 engaged subscribers. Monitor bounce rates and spam complaints daily.

Week 3: Expand to 25,000. Continue monitoring.

Week 4: Full volume if metrics are clean (bounce rate under 1%, spam complaints under 0.05%, inbox placement above 95%).

If at any point metrics go bad, pause and diagnose. Do not push through. Bad signals compound.

Do not switch everything at once. Run both ESPs in parallel during the warm-up period.

Automation flows stay on the old ESP until the new one is warmed up and flows are rebuilt.

Campaigns shift to the new ESP following the warm-up schedule. Once you are at full volume on the new ESP, sunset the old one.

This overlap costs you two ESP subscriptions for 4-6 weeks. That cost is much less than the revenue you lose from a botched migration.

After the migration is complete, verify: all automation flows are active and triggering correctly, subscriber counts match between old and new ESP (minus cleaned contacts), DNS records are correct and passing authentication checks, inbox placement is at or above pre-migration levels.

Monitor closely for 30 days after full migration. Some issues only surface once you are at full volume. Keep the old ESP account active (but not sending) as a backup for 60 days.