SMTP Error 500 - Syntax Error / Command Unrecognized

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What This Error Means

SMTP error 500 indicates a fundamental protocol problem:

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Invalid Command
The mail client sent a command the server doesn't understand.
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Software Bug
A bug in your email software is sending malformed commands.
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Protocol Mismatch
Your mail server and the recipient's server have incompatible protocol versions.

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Ensure your mail server software is up to date with the latest patches.

Review your mail server configuration for any custom commands or extensions that might not be supported.

If the issue persists, contact your mail server administrator to investigate the SMTP logs.

500 5.5.1 Command unrecognized
500 Syntax error, command unrecognized
500 Error: bad syntax