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.