The correct option for the protocol primarily used for sending email messages between servers is B. SMTP (Simple Mail Transfer Protocol). Detailed Explanation:

1. Understanding SMTP:
2. SMTP is a protocol specifically designed for sending emails across the Internet. It operates on a client-server model where the email client (the sender) communicates with the email server (the recipient's server) to transfer messages.
3. SMTP works by establishing a connection between the sender's email server and the recipient's email server, allowing the transfer of email messages. It uses a set of commands and responses to facilitate this communication.
4. How SMTP Works:
5. When you send an email, your email client (like Outlook or Gmail) connects to your email server using SMTP.
6. The email is then sent to the recipient's email server, which stores it until the recipient retrieves it.
7. SMTP typically operates over TCP (Transmission Control Protocol) on port 25, although it can also use ports 587 and 465 for secure connections.
8. Why Other Options Are Incorrect:
9. A. HTTP (Hypertext Transfer Protocol):
   • HTTP is used for transferring web pages and other resources on the World Wide Web. It is not designed for sending emails. While webmail services (like Gmail) may use HTTP to allow users to access their emails through a web browser, the actual sending of emails between servers is handled by SMTP.
10. C. FTP (File Transfer Protocol):
    • FTP is used for transferring files between computers on a network. It is not related to email transmission. FTP is primarily used for uploading and downloading files, not for sending or receiving email messages.
11. D. POP3 (Post Office Protocol version 3):
    • POP3 is a protocol used by email clients to retrieve emails from a mail server. It allows users to download their emails to their local devices. While POP3 is essential for accessing emails, it does not send emails between servers; that function is performed by SMTP.

Summary of Key Points:

• SMTP is the protocol used for sending emails between servers.
• It operates on a client-server model and uses TCP for reliable communication.
• HTTP is for web pages, FTP is for file transfers, and POP3 is for retrieving emails, not sending them.

Revision Summary:

• SMTP is the correct protocol for sending emails between servers.
• It operates over TCP, typically on port 25.
• HTTP is for web content, FTP is for file transfers, and POP3 is for email retrieval.
• Understanding the specific functions of each protocol is crucial for distinguishing their roles in email communication.