Email Deliverability After an ESP Migration: How to Avoid Losing Your Reputation

When you switch ESPs, you often get new sending IPs. Those IPs have no reputation — or worse, they may have reputation from previous tenants who used them badly.

Additionally, new authentication configuration introduces risk: SPF records must be updated, DKIM keys must be set up in the new ESP, and everything must align for DMARC. A single misconfiguration can break delivery for your entire list.

1. 1

   Audit current ESP configuration

   Document all DKIM selectors, SPF includes, sending IPs, and sending domains in the old ESP. Export complete subscriber list with engagement timestamps and suppression list.

2. 2

   Configure new ESP authentication

   Set up DKIM in the new ESP and publish the public key to DNS BEFORE switching traffic. Update SPF to include both old and new ESP during the transition period. Verify DMARC alignment.

3. 3

   Warm up new sending IPs

   Start with your most engaged segment (opened in last 30 days). Send 10-20% of volume through new ESP in weeks 1-2. Monitor bounce rates and inbox placement closely before increasing.

4. 4

   Run parallel systems

   Don't cut over all at once. Increase new ESP share gradually: 20% → 50% → 100% over 4-6 weeks. Keep old ESP available as fallback if problems emerge.

5. 5

   Complete DNS switchover

   Once 100% traffic moves to new ESP, remove old ESP from SPF record. Keep old DKIM selectors active for 30+ days (emails in transit may still reference them).

6. 6

   Double-monitor for 30 days post-migration

   Check blacklist status daily for new sending IPs. Compare bounce rates and complaint rates week-over-week between old and new. Watch Google Postmaster domain reputation closely.

• ·PRE: Export subscriber list with engagement history and suppression list
• ·PRE: Document all DKIM selectors active in current ESP
• ·PRE: Update SPF to include new ESP (keep old ESP in SPF during transition)
• ·PRE: Set up DKIM in new ESP and publish public key to DNS
• ·PRE: Segment list by engagement level for warm-up sequence
• ·DURING: Monitor bounce rate — pause if above 2% on new ESP
• ·DURING: Monitor spam complaint rate — pause if above 0.1%
• ·POST: Verify DKIM and DMARC passing in new ESP reports
• ·POST: Remove old ESP from SPF after traffic fully migrated
• ·POST: Keep old DKIM selector in DNS for 30+ days after switchover
• ·POST: Monitor Google Postmaster domain reputation for changes

1. Export complete subscriber list with engagement history from old ESP 2. Document all DKIM selectors in your old ESP's configuration 3. Verify SPF record includes both old and new ESP (for the transition period) 4. Set up DKIM in the new ESP and add the public key to DNS before switching 5. Set up DMARC reporting in the new ESP 6. Segment your list by engagement level for the warm-up sequence 7. Schedule the migration for a low-volume period (not during a big campaign)

Don't switch cold — run old and new ESPs in parallel:

Week 1-2: Send from old ESP as normal. New ESP is configured but not yet sending. Week 3-4: Send 10-20% of volume from new ESP to your most engaged segment. Monitor bounce rates, delivery, and inbox placement. Week 5-6: Increase new ESP to 50% if metrics look good. Continue decreasing old ESP. Week 7-8: Complete transition to new ESP.

This gradual approach warms up the new sending IPs with your best traffic first, establishing good reputation before exposing the new IPs to your less-engaged list.

Double monitoring intensity for 30 days after migration:

- Check blacklist status daily for new sending IPs - Monitor Google Postmaster Tools for domain reputation changes - Compare bounce rates week-over-week between old and new ESP - Compare spam complaint rates between platforms - Verify DKIM and DMARC are passing in the new ESP's reports

If you see reputation problems in the first two weeks, pause migration and investigate before continuing.