A domain registered last week that sends a few hundred cold emails on day one looks exactly like a spammer. Mailbox providers have no history for it, so they start cautious.

Warm-up means building that history gradually, sending small daily volumes to engaged recipients and raising them week by week while you watch bounces and complaints.

uChecker's domain and IP warm-up guide includes a four-week schedule with volume targets and the mistakes that usually undo the progress.