SMTP stands for Simple Mail Transfer Protocol. It is a standard communication protocol used for sending and receiving electronic mail (email) messages between servers, clients, and other email systems. SMTP is primarily responsible for the transfer of outgoing emails from the sender's email client or server to the recipient's email server. It provides a reliable and efficient way to route and deliver email messages across the internet or within a local network.

SMTP operates in a client-server model, where the client initiates the connection to the server to send an email. The client establishes a connection to the SMTP server on port 25 (or other designated ports for secure variants like SMTPS or STARTTLS), and then it communicates with the server to deliver the email message along with recipient information.

SMTP defines the rules and format for composing email messages, including the sender and recipient addresses, subject, message body, and any attachments. After receiving the email, the recipient's email server can either deliver it to the recipient's mailbox or forward it to other servers if needed.

SMTP also handles error messages and notifications related to email delivery, such as bounce-back messages if an email couldn't be delivered successfully.

In summary, SMTP is a crucial component of the email infrastructure, facilitating the transmission of email messages across different servers and networks in a standardized and reliable manner.