IP warm-up

What it means

AWS explains why: providers consider the reputation of the sending IP, and mail from addresses with little or no history may go to junk or be blocked. Warming up means increasing volume from the new address gradually before using its full capacity. AWS says some providers form a positive reputation in around two weeks and others take up to six, and advises sending first to your most active users and slowing down when blocking or throttling notices appear.

Google gives the same advice for any volume increase: start low with engaged users, send at a consistent rate without bursts, and if messages bounce or are deferred, reduce volume until errors fall, then increase again.

On a shared IP the provider keeps volume steady across customers, which M3AAWG lists among the reasons small senders use shared environments.