EC
500
SMTPERRORCommonClient ErrorHIGH confidence

Syntax error, command unrecognized

What this means

The 500 code means the server could not understand the command that was sent because it was not a valid SMTP command. This is almost always a client-side software bug.

Why it happens
  1. 1The client sent a misspelled command (e.g., `HELO` instead of `EHLO`).
  2. 2There was a data transmission error that corrupted the command.
  3. 3The client is not compliant with the SMTP protocol.
How to reproduce

A misconfigured email client sends a malformed command.

trigger — this will error
trigger — this will error
MEIL FROM:<sender@example.com>
500 5.5.2 Syntax error, command unrecognized

expected output

500 Command unrecognized

Fix

Fix client software

WHEN When you are developing the SMTP client

Fix client software
// Ensure all commands sent to the server are RFC-compliant
const command = "MAIL FROM:<test@example.com>"; // Correct
const badCommand = "MEIL FROM:<test@example.com>"; // Incorrect

Why this works

The server can only act on valid commands. Fixing the client is the only solution.

What not to do

✕ Retry the same command

The command is fundamentally invalid. Retrying will produce the same error and waste resources.

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