501 Syntax error in parameters or arguments

The server understood the command but not what came after it, most often an email address written in a way it does not accept.

501 Syntax error in parameters or arguments

Permanent: retrying will not help

The server understood the command but not what came after it, most often an email address written in a way it does not accept.

Usual causes

A malformed email address, such as one with a space, a missing @, or extra characters.
A sender address (MAIL FROM) that is empty or invalid where the server needs one.

What to do

Check the address that was rejected. Remove spaces, angle brackets and quotes you added by accident.
Make sure the From address is a complete, real address on a domain you control.

Usually the fault of: sender.

Common questions.

What does 501 mean?

The server understood the command but not what came after it, most often an email address written in a way it does not accept.

Will retrying fix 501?

No. It is a permanent failure, so retrying the same message will not help, and repeated attempts can hurt your sending reputation. Fix the cause first.

How do I fix 501?

Check the address that was rejected. Remove spaces, angle brackets and quotes you added by accident. Make sure the From address is a complete, real address on a domain you control.

Other codes: 250 · 421 · 450 · 451 · 452 · 500 · 503 · 530 · Decode a whole bounce message

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