Which protocol is commonly used to send email messages between servers?

The correct protocol for sending email messages between servers is the Simple Mail Transfer Protocol (SMTP). SMTP is specifically designed for the transmission of email over the internet. It functions by facilitating the sending and routing of email messages from the sender's email server to the recipient's email server. This protocol is essential for handling outgoing mail and plays a critical role in the email delivery process.

SMTP operates primarily on the application layer and uses well-defined commands and responses to ensure reliable communication between servers. When an email is sent, the sender's server establishes a connection with the recipient's server using SMTP and transmits the message until it reaches its destination.

In contrast, other protocols mentioned serve different purposes. The File Transfer Protocol (FTP) is used for transferring files between computers but is not suited for email transmission. The Hypertext Transfer Protocol (HTTP) primarily handles communication for the web, allowing the retrieval of web pages but not for sending emails. The Internet Message Access Protocol (IMAP), while involved in email as well, is used for retrieving and managing messages from a server rather than sending messages between servers.