SMTP Error Codes & Meanings

What do 4xx SMTP error codes mean? (Temporary failures) What do 5xx SMTP error codes mean? (Permanent failures) What are common 4xx codes and their meanings? (e.g., 421, 451) What are common 5xx codes and their meanings? (e.g., 550, 553, 554) Where can I find a list of SMTP error codes? What are SMTP reply codes? What does 421 mean? What does 450 mean? What does 451 mean? What does 452 mean? What does 454 mean? What does 500 mean? What does 501 mean? What does 502 mean? What does 503 mean? What does 504 mean? What does 550 mean? What does 551 mean? What does 552 mean? What does 553 mean? What does 554 mean? What is a “custom” SMTP code? Why do different ISPs use non-standard codes? How do enhanced status codes (X.Y.Z) work? How can you interpret extended SMTP responses (e.g., 5.7.1)? What’s the difference between 5.1.1 and 5.2.2? How can error codes indicate throttling vs policy rejection? What are typical Gmail vs Microsoft error code patterns?