500 5.5.2 Syntax error in command
Meaning
The receiving server could not recognize the SMTP command sent by your mail server due to a syntax error or unsupported feature.
Common causes
- Your mail server or application is sending malformed SMTP commands.
- A firewall or proxy is corrupting the SMTP traffic in transit.
- There is a severe incompatibility between the sending and receiving mail server software.
How to fix
- Check your application's SMTP configuration and ensure it follows RFC standards.
- Inspect network firewalls or proxies that might be altering outbound SMTP traffic.
- Capture a packet trace or SMTP debug log to see exactly what malformed command is being sent.
Example bounce
500 5.5.2 Syntax error, command unrecognized.
FAQ
- Does this mean the email address is bad?
- No, this is a protocol-level error between the two servers communicating, not an issue with the recipient's address.
- How do I get an SMTP trace?
- Most mail libraries (like PHPMailer, Nodemailer, or Python's smtplib) have a debug mode that prints the raw SMTP conversation.