What it means
Email warm-up builds a new inbox's sending reputation by starting with a few emails a day and raising the number slowly. Warm-up tools send messages between a network of real inboxes that open them, reply to them and move them out of spam. That tells mailbox providers the sender is trustworthy.
A brand-new domain sending at full volume on day one looks like a spammer. Warm-up usually keeps running at a low level after the campaign starts, to hold the reputation steady. It cannot rescue a bad list: bounces and spam complaints spend the reputation warm-up built.
How we use it
The infrastructure is built before any campaign, because warm-up takes weeks you cannot buy back later. On our timeline the domains and inboxes are set up and warmed in weeks 3 to 6, before the first campaigns go live.
The warm-up presets we use for each sending provider are in our cold email deliverability checklist.