SMTP (Simple Mail Transfer Protocol) is the correct answer for the protocol used for sending and receiving email messages. SMTP is a standard communication protocol that enables the transmission of electronic mail over the internet.

Explanation:

1. Introduction to SMTP:
SMTP is a protocol that defines the rules and standards for sending and receiving email messages. It was first defined in 1982 by Jon Postel and his colleagues, and it has been widely adopted as the primary protocol for email communication.

2. How SMTP works:
SMTP operates in a client-server architecture. When an email is sent, the client (usually an email application) initiates a connection to the email server using the SMTP protocol. The client then submits the email to the server, which processes it and forwards it to the recipient's email server. The recipient's email server then uses the appropriate protocol (such as POP3 or IMAP) to deliver the email to the recipient's email client.

3. SMTP Commands:
SMTP uses a set of commands to facilitate the communication between the client and the server. Some of the commonly used commands include:
- HELO/EHLO: Used to initiate the SMTP session and identify the client.
- MAIL FROM: Specifies the sender of the email.
- RCPT TO: Specifies the recipient(s) of the email.
- DATA: Indicates the start of the email message.
- QUIT: Terminates the SMTP session.

4. Additional Features:
SMTP also supports additional features such as authentication, encryption, and delivery status notifications. Authentication is used to verify the identity of the sender, while encryption ensures the confidentiality and integrity of the email during transmission. Delivery status notifications allow the sender to receive notifications about the delivery status of their emails.

5. Alternative Protocols:
While SMTP is the primary protocol for sending and receiving email messages, there are alternative protocols such as POP3 (Post Office Protocol version 3) and IMAP (Internet Message Access Protocol) that are used for retrieving emails from the server. POP3 is more commonly used for downloading emails to a local client, while IMAP allows for accessing and managing emails directly on the server.

In conclusion, SMTP is the protocol used for sending and receiving email messages. It defines the rules and standards for email communication and operates in a client-server architecture. SMTP commands are used to facilitate the communication between the client and the server.