Which of the following protocols is primarily concerned with the sending and receiving of mail?

The protocol primarily concerned with the sending and receiving of mail is SMTP, which stands for Simple Mail Transfer Protocol. This protocol is specifically designed for transferring email messages between servers. SMTP operates on a client-server model, where the client sends a request to the server to deliver an email to a recipient. It's a fundamental protocol for email as it handles the sending part of the communication, ensuring that messages are routed appropriately until they reach the recipient's mail server.

In contrast, other protocols serve different purposes. HTTP (Hypertext Transfer Protocol) is used for transferring web pages and resources over the internet, focusing on the delivery of content rather than mail services. FTP (File Transfer Protocol) is utilized for transferring files between computers and is not specifically related to email communications. SSH (Secure Shell) is designed for secure remote access and command execution on networked devices, rather than handling mail functions. Thus, SMTP is the correct answer for this question related to email communication.