$ man domain-warming

Domain Warming

Gradually increasing sending volume from a new domain so email providers trust it and don't flag messages as spam.

If you register a domain today and send 500 emails tomorrow, they all land in spam. Email providers don't know you. You have no reputation. Domain warming builds that reputation by starting slow (10-20 emails/day), engaging with real inboxes (automated or manual), and scaling up over 2-4 weeks. By the time you hit full volume, the domain looks legit.

Instantly has built-in warming. I enable it for every new sending domain. It sends low-volume emails between "real" mailboxes in the warming pool, replies are automated, and the system gradually ramps volume. I don't touch it. It just runs. After 3-4 weeks, I route real campaign traffic to that domain. If I skip warming, deliverability tanks immediately.