IP warm-up is the practice of gradually increasing the number of emails sent from a new IP address so that mailbox providers can learn to trust it. A new IP has no sending history. If it suddenly sends 100,000 emails, providers see a pattern that matches spam runs and respond by throttling, deferring or filtering the mail. Warming up gives them time to observe good behaviour.
A typical warm-up starts with a few hundred or a few thousand emails a day and increases by a steady percentage, often between 20% and 50% a day depending on results, until it reaches normal volume. The first sends should go to your most engaged contacts, such as recent clickers and buyers, because their positive engagement is what builds reputation. Old, cold contacts belong at the very end, if at all.
Watch the signs as you go. Rising soft bounces, “try again later” deferrals, unusually slow delivery to one provider, or a sudden drop in opens at one provider all mean slow down and hold the current level for a few days. Providers judge reputation separately, so you may move faster at one than at another.
IP warm-up mainly matters for dedicated IPs. On a shared IP pool, the platform manages IP reputation, and your concern is mostly domain reputation, which needs similar care when the domain is new. Warm-up also applies after a long pause: an IP that hasn’t sent for a few months may need a short ramp again.
A Chennai travel company gets a dedicated IP for its 300,000-contact list. Day one: 1,000 emails to people who booked in the last 60 days. It increases volume by about a third each day, holds for three days when Outlook defers some mail in week two, and reaches full volume in five weeks.
MailNesto lets you set a warm-up for each sending server, using a ready-made ramp, a start-and-increase formula or your own day-by-day caps, alongside a daily sending limit.