Domain warming is the way you establish a sending reputation for a new domain, or a domain that has not been used to send email recently. The warm-up process involves sending email from the new domain starting with small volumes, and gradually increasing the volume of email each day or week according to a set schedule.
Mailbox providers are suspicious of email coming from a domain that has no sending reputation or that has an unfamiliar sending pattern. They have to protect their customers from spam, so they will block or bulk large volumes of email from a new domain because it is most often coming from spammers.
Warming a domain develops your domain's identity and sending reputation, which helps gain the mailbox providers’ trust.
Read the How to warm up a domain article for more details.
