The protocol used to send email between mail servers is SMTP.
SMTP stands for Simple Mail Transfer Protocol. It is an application-layer protocol that moves messages from a sending mail server to a recipient’s mail server, often through one or more intermediate relays. Servers commonly use TCP port 25 for server-to-server transfer, while authenticated message submission commonly uses ports 587 or 465.
SMTP is a push and delivery protocol, not normally a mailbox-reading protocol. After a message reaches its destination server, an email application generally retrieves it with IMAP or POP3. This distinction is the main source of confusion in questions that list all three protocols together.
SMTP grew out of early ARPANET email work and was standardized in the early 1980s. Modern deployments add extensions for authentication, encryption, larger or richer messages, and internationalized email addresses, but the basic mail-transfer role remains the same.