Domain warmup is the practice of gradually increasing sending volume from a new email domain so mailbox providers learn to trust it. Skipping it is the single fastest way to land cold outbound in spam.
Why new domains need it
A domain with no sending history has no reputation, so a sudden burst of cold email looks exactly like a spammer. Warmup starts with a low daily volume, often to engaged or seed inboxes, and ramps over weeks while replies and opens build a positive track record.
Warmup and its limits
Warmup builds reputation, but it does not survive bad behavior: high bounces, spam complaints, or a volume spike can undo it quickly. It works together with authentication and good list hygiene to protect deliverability. Many teams also send new outbound from a separate domain to shield the primary brand domain.