SMTP, the Simple Mail Transfer Protocol, is the internet standard for sending email. It governs how a mail client hands a message to a server and how servers relay messages to one another until they reach the recipient's mail system. Every email you send passes through at least one SMTP conversation.
An SMTP session is a text-based exchange of commands and numeric response codes. The client greets the server with EHLO, declares the envelope sender with MAIL FROM, names each recipient with RCPT TO, and transfers the message with DATA. The server replies with codes: 2xx for success, 4xx for temporary failures, and 5xx for permanent ones.
SMTP handles sending and relaying only - it does not retrieve mail from a mailbox, which is the job of IMAP or POP3. Modern SMTP runs over encrypted connections, typically on port 587 with STARTTLS for client submission or port 465 for implicit TLS, while server-to-server relay traditionally uses port 25.
Understanding SMTP response codes is practically useful because they explain exactly why a message succeeded or failed. A 550 tells you an address is permanently invalid (a hard bounce), while a 421 or 452 signals a temporary problem (a soft bounce). Email verification tools use a partial SMTP handshake to probe mailbox existence without delivering a message.