IP warming is the process of building sending reputation on a brand-new IP address or domain by starting with a small volume of email and increasing it steadily over days and weeks. A fresh IP has no history, and mailbox providers treat unknown senders with suspicion, so a sudden burst of volume looks exactly like a spammer.
The mechanics are simple in principle: begin with a modest number of messages sent to your most engaged recipients, then roughly double the daily volume each day or every few days as long as engagement stays high and complaints stay low. Warming establishes a track record that tells providers your mail is wanted.
Engagement is the fuel of warming. Sending early volume to people who open, click, and reply teaches providers that your mail belongs in the inbox. Warming with a cold, unengaged, or unverified list backfires - the bounces and complaints define your new reputation as bad from the start.
The same principle applies to warming a new sending domain, which increasingly matters more than the IP. A typical warm-up spans two to six weeks depending on target volume. Rushing it, or resuming full volume after a long pause without re-warming, are the classic mistakes that land senders in spam.