How do you migrate from one ESP to another safely?

ESP migration requires careful planning to avoid deliverability disruption. Rushing causes problems; methodical transition protects your reputation.

Pre-migration:

Audit current setup: authentication records, sending domains, tracking domains, suppression lists

Document everything: templates, automations, integrations, API usage

Export suppression lists: bounces, unsubscribes, complaints (critical)

Parallel running phase:

Set up new ESP completely before switching traffic

Verify authentication (SPF, DKIM, DMARC) works correctly

Test thoroughly with seed lists

Keep old ESP active as backup

Traffic migration:

Start with small percentage of traffic to new ESP

Monitor deliverability closely

Gradually increase new ESP traffic over weeks

Warm new IPs if dedicated

Don't rush; respond to signals

DNS transition:

Update SPF to include both ESPs during transition

Add new DKIM records before sending from new ESP

Only remove old ESP from records after full migration

Post-migration:

Verify all automations work correctly

Confirm tracking and reporting function

Keep old ESP accessible for historical data

Update documentation

Change ships in calm waters, not during a storm. Plan migrations during low-volume periods when possible.