A standard communication protocol used for sending and receiving email over the internet. SMTP facilitates the transfer of email messages from a client to an email server and between email servers. It defines the rules and procedures for how email is transmitted from one server to another, ensuring that messages are correctly routed and delivered to the recipient's email server. SMTP operates by establishing a connection between the sender's and recipient's servers, and it uses a series of commands and responses to manage the sending process. While SMTP is primarily responsible for sending email, it is often used in conjunction with other protocols like POP3 or IMAP, which handle receiving and retrieving messages.