Mailboxes

A brand-new address that suddenly sends a lot of cold email gets filtered as spam. Warmup builds trust gradually: each day the mailbox is allowed a few more emails (2 on day 0, then roughly 5 to 6 more per day) until it reaches the daily volume you set, so 40/day takes about a week. Warmup traffic is sent between your own mailboxes, so it needs at least two with warmup on. That daily limit also caps sequence sends. Health tracks bounces and spam complaints and pauses a mailbox that turns risky.