Domain warm-up
Warm-up is increasing sending volume gradually on a new domain, so mailbox providers build a reputation for it before it ever sends at full rate.
A domain with no history that suddenly emits 500 messages looks exactly like a domain bought that morning for spam, because that is the most common thing that pattern is. Starting at ten or twenty a day and roughly doubling each week over four to six weeks reads as normal business mail.
Two rules make the difference. Send cold mail from a separate domain, usually a close variant of your main one, so a bad campaign cannot take your invoices and password resets down with it. And keep the ramp slow even when pipeline is thin, because the temptation to skip it arrives exactly when the cost of skipping it is highest.
Goes deeper: Why Your Cold Emails Land in Spam