What is Email warm-up?
Email warm-up is the practice of raising a new inbox's or domain's sending volume gradually, so mailbox providers build a reputation for it before it sends at full volume.
Also called: inbox warm-up, domain warm-up.
A brand-new domain has no history. Mailbox providers judge mail partly by the sender's past behaviour, and a domain that goes from nothing to hundreds of messages a day looks like a compromised account or a spammer. Sudden jumps in volume are exactly what filters watch for.
Warm-up spreads that growth over weeks: a small number of sends a day at first, raised step by step while bounces, complaints and replies stay healthy. Many senders also run automated warm-up networks that exchange messages between inboxes; how much those help is debated, and none replaces sending wanted mail to a clean list.
Warm-up does not fix a bad list. Sending a warmed inbox to unverified addresses spends the reputation you built; see bounce rate. Nor does it fix authentication: SPF, DKIM and DMARC should be in place before the first send.
The email warm-up schedule planner builds a day-by-day ramp from your target volume and number of inboxes, and flags when your full volume crosses Gmail's 5,000-a-day bulk sender threshold.