501: syntax error in arguments
The command was recognised, but what came with it was malformed, often an address or a host name the server cannot accept.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 501
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
501 Syntax error in parameters or arguments
What it means
With 501 the server knows the command but rejects its arguments: a badly formed address in MAIL FROM or RCPT TO, an invalid name in HELO or EHLO, or an argument given to a command that takes none. RFC 5321 requires a 501 when a command uses characters that are not allowed in a domain name, such as the character _ in the HELO name. It is permanent for that command. The enhanced code next to it, such as 5.1.3, 5.1.7 or 5.5.2, tells you which part was wrong.
Common causes
- A malformed address, such as one without an @ or with unbalanced angle brackets.
- A HELO or EHLO host name with characters that are not allowed in domain names, such as _.
- Arguments sent with a command that takes none.
How to fix it
- 01Check the address or host name quoted in the reply.
- 02Make sure your software writes MAIL FROM:<address> and RCPT TO:<address> with angle brackets.
- 03Set the sending server's HELO name to a full host name made of letters, digits, hyphens and dots.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- RFC 5321, section 4.2.3: Reply Codes in Numeric Orderrfc-editor.org
- RFC 5321, section 4.1.2: Command Argument Syntaxrfc-editor.org
Related codes
- 5.1.3Recipient address is malformed.The recipient address breaks the rules for how an email address must be written, so the server refused it before checking whether it exists.
- 5.1.7Sender address is malformed.The sender address in the envelope (MAIL FROM) is written in a way the receiving server cannot accept, so it refused the message.
- 5.5.2SMTP syntax error.A command from the sending software could not be understood, because its syntax was wrong or the command was not recognised at all.
- 5.5.4Invalid command arguments.A valid SMTP command was sent with an argument the server does not accept, because the value was out of range or named a feature it does not know.
- 500Command not recognised.The server did not recognise the command it received, or the command line was too long. This is a permanent error in the SMTP conversation.
All 105 standard codesEvery code in the wikiPaste your own bounce