SMTP is the protocol used to send email between servers. Learn what SMTP does, how sending works, and why SMTP errors matter.
SMTP (Simple Mail Transfer Protocol) is the standard protocol used to send email from one server to another. When you click send, your email client hands the message to an SMTP server, which then relays it toward the recipient’s mail server.
SMTP is responsible for the server-to-server transfer and includes responses that indicate whether a message was accepted, delayed, or rejected. Many delivery issues show up as SMTP status codes and bounce reasons.
Understanding SMTP helps when debugging bounces, blocks, and throttling. It also matters for tooling like email verification and deliverability checks, where the difference between temporary and permanent SMTP responses changes how you should react.