Which protocol is used for sending mail from a client to a server?

The protocol used for sending mail from a client to a server is SMTP, which stands for Simple Mail Transfer Protocol. This protocol is specifically designed for the purpose of transferring emails over the internet. When a user composes an email and hits the send button, the email client communicates with the mail server using SMTP to deliver the message to the recipient's mail server.

SMTP operates on a push model, meaning that it sends messages from the client to the server. It is optimized for this one-way communication, facilitating the batch sending of email messages and handling various functions such as queuing messages until an SMTP connection can be established with the recipient's server.

On the other hand, other protocols listed serve different purposes: POP3 (Post Office Protocol 3) and IMAP (Internet Message Access Protocol) are primarily used for retrieving emails from a server to a client, while FTP (File Transfer Protocol) is meant for transferring files between computers, and HTTP (Hypertext Transfer Protocol) is used for transferring web pages and resources over the web. Therefore, SMTP's specific role in sending emails makes it the correct choice in this context.