How long should you keep both ESPs active during transition?

Running both ESPs simultaneously provides safety margin during transition. Don't cancel the old ESP too quickly.

• Minimum parallel period: 4-8 weeks
• Week 1-2: New ESP set up, testing, initial warmup
• Week 3-4: Gradual traffic shift, monitoring closely
• Week 5-6: Majority of traffic on new ESP, old ESP handling remainder
• Week 7-8: Final migration, verification, documentation

Why keep the old ESP active:

• Rollback capability: If new ESP has problems, you can shift traffic back
• Warmup buffer: Old ESP handles volume while new IPs warm
• Data access: Historical reports, tracking data, suppression sync
• Link preservation: Old tracking links in sent emails continue working

When to deactivate old ESP:

• All traffic successfully moved
• New ESP metrics stable and healthy
• Historical data exported or no longer needed
• Tracking links in old emails expired or redirected
• Confident you won't need rollback

After deactivation:

• Remove old ESP from SPF record
• Keep DKIM records for a while (signed emails may still be forwarded)
• Archive exported data
• Document the completed migration
• Don't scuttle the old ship until the new one has proven seaworthy.