A warm-up is a crucial period where a sender’s identity and reputation is established on the new domain(s) and/or IP(s). ISPs and mailbox providers give senders a certain period to prove their legitimacy when sending from new domains or IPs. This is called the warm-up period. If the sender fails to establish themselves as legitimate, the ISP or mailbox provider will block or bulk their emails. A warm-up may last for up to six weeks, and during this period, senders need to send a slow ramp of volume to highly-engaged subscribers.
You may also read: How to Warm Up Your Dedicated Email IP?