Outbound Glossary
Warm-up
Building a sending reputation by starting small and growing slowly. Skipping it does not slow a campaign down, it ends one.
A new domain and a new mailbox have no reputation, and a mailbox provider treats an unknown sender that suddenly sends hundreds of messages as a spammer, because that is what the pattern looks like. Warm-up means starting at a handful of messages a day and increasing over weeks, so a history exists before the volume does.
What actually builds the reputation is engagement, not the count. Messages that get opened and replied to teach the provider you are wanted; messages that get deleted or marked as spam teach the opposite, faster. So a warm-up that only sends into automated seed accounts builds much less than it appears to.