$ man minimum-warmup-period

Minimum 2-Week Warmup

The minimum time a new domain and mailbox must spend in warmup before sending real campaign emails. Two weeks of gradually increasing volume with engagement signals (opens, replies) from the warmup pool.

New domains have zero reputation. Email providers don't know if you're legitimate or a spammer. Warmup builds that reputation by proving your emails get opened, replied to, and don't bounce. Two weeks is the floor — not the ceiling. Some domains need 3-4 weeks, especially on Microsoft 365. Sending real campaigns from a domain that hasn't warmed is the fastest way to burn it.

I enable Instantly's built-in warmup on every new mailbox from day one. The warmup pool sends low-volume emails between real mailboxes, generates opens and replies, and gradually increases volume. I don't touch the mailbox during warmup — no manual emails, no test sends, nothing. After 2 weeks, I check the warmup stats. If the domain shows healthy engagement and no flags, I start routing low-volume campaign traffic to it. If anything looks off, I extend warmup another week.