What is IP/domain warming after migration?
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You've moved to a new ESP. New IP addresses, possibly a new sending subdomain too. Before you send your first campaign to your full list, you need to think about warming, because new infrastructure means zero history, and zero history means mailbox providers will be skeptical.
IP warming is the process of gradually building reputation on new sending IPs. Mailbox providers track every IP that sends them mail. A brand-new IP that suddenly sends 100,000 emails looks exactly like what spammers do when they spin up new infrastructure after getting blocked. So even if you're a perfectly legitimate sender with years of history, your new IPs start from scratch.
The warmup process works by starting with a small volume of your most engaged subscribers, people who regularly open and click, and gradually increasing over several weeks. The positive engagement signals (opens, clicks, no complaints, low bounces) tell mailbox providers "this IP sends to people who want the mail." As you establish that track record, you can send higher volumes and reach less-engaged segments.
Domain warming follows the same logic if you're switching to a new sending subdomain. If you've been sending from mail.yourdomain.com and moving to newsletter.yourdomain.com, that subdomain has no history either. Start smaller and build up.
Most ESPs offer pre-configured warmup schedules that increase volume automatically across a recommended timeline. Use them. If yours doesn't, there are widely-available warmup schedule templates that map out daily volume targets by week for different starting list sizes. The main thing is: don't rush. Warming takes 30 to 60 days for most senders. That's the price of a clean migration.
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