A brand-new sending IP or domain has no history with mailbox providers, so sending at full production volume immediately reads as spam-like regardless of content. Warm-up ramps volume over roughly two to four weeks, starting with the most engaged recipients first.
A typical schedule doubles daily or every few days rather than jumping straight to steady-state traffic, while watching bounce and complaint rates at every step. A hard-bounce rate above roughly 2% or a complaint rate above 0.1% during warm-up is a signal to pause and clean the list, not push through.
Warm-up applies separately to a new domain and a new dedicated IP; migrating providers with an existing domain but a brand-new IP still needs an IP-side ramp even though the domain's own reputation may carry over in part.