What protocol is commonly used for sending emails?

Simple Mail Transfer Protocol (SMTP) is the standard protocol used for sending emails across the Internet. It is designed primarily for transmitting messages from a sender's email client to an email server, as well as between email servers. SMTP operates over the TCP/IP protocol suite, utilizing specific ports such as port 25, port 587, or port 465 for secure transmission.

When an email is composed and sent, it is formatted using SMTP to ensure the message gets to the desired recipient's mail server. SMTP efficiently handles outbound mail delivery, making it an essential component of email communication. While other protocols like POP and IMAP are involved in retrieving and managing emails on a user's device, they do not govern the sending process.

Protocols like HTTP are utilized for web communication and do not support direct email transmission. Similarly, FTP is meant for transferring files, not messages. Thus, SMTP is specifically tailored for the purpose of sending emails, making it the correct answer.