SMTP error codes are the most precise diagnostic signal available when an email fails to deliver, but most email marketers never learn to read them fluently. This guide covers every major code across the 2xx, 4xx, and 5xx series, explains enhanced status codes in the X.Y.Z format, and covers ISP-specific variations for Gmail, Outlook, and Yahoo that behave differently from standard codes. Includes an ESP bounce label translation map, a retry versus suppress decision framework for every code, and a complete quick reference table for use alongside any bounce report.