How long should you keep both ESPs active during transition?
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The temptation to cancel your old ESP the day you go live on the new one is real, especially when you're paying for two platforms simultaneously. Resist it. Running both ESPs in parallel for several weeks is cheap insurance against the things that tend to go wrong after a migration.
A reasonable parallel period is four to eight weeks, depending on your complexity. Here's roughly how that breaks down. The first two weeks are for setup and testing: getting authentication configured, templates rebuilt, suppression lists imported, and test sends verified. Nothing goes to your full list yet.
Weeks three and four: start routing a small fraction of your live traffic to the new ESP. Your most engaged subscribers are a good starting point since they're the best candidates for IP warmup and you'll get fast feedback on deliverability. Watch your logs, monitor your open rates, and check your Google Postmaster domain reputation data closely. Weeks five and six: shift the majority of your sending to the new ESP. The old ESP handles only residual traffic and serves as a fallback if something unexpected happens. By this point you should have enough data to evaluate whether the migration is going well.
Only after you're confident, everything delivering cleanly, metrics stable, no unusual bounce spikes, should you cancel the old account. Make sure to download a final export of all data, including full history and suppression lists, before you do. You won't be able to access anything after cancellation.
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