Definition
SMTP, or Simple Mail Transfer Protocol, is a standard internet protocol used for sending electronic mail messages between servers. It defines how email clients and servers should communicate to transmit outgoing mail, acting as the primary mechanism for email delivery across the internet. SMTP operates on a push model, where a sending server connects to a receiving server to transfer the message. Understanding SMTP is crucial for network administrators and security professionals to ensure reliable email delivery and troubleshoot communication issues. For instance, when you send an email from your Gmail account, SMTP is the protocol that facilitates its transfer to Google's mail servers and then to the recipient's mail server. It is distinct from POP3 or IMAP, which are used for retrieving mail. This protocol is frequently covered on CompTIA Security+, Network+, and the CISSP certification exams.