SMTP error 501 is a permanent or "5xx" error message returned by a mail server to indicate that the email delivery request is malformed or contains syntax errors. Unlike transient errors (4xx codes), SMTP error 501 signifies that the email server will not accept or process the email message due to fundamental issues with the message's format or content.
What's Causing This Error?
SMTP error 501 is typically caused by one or more of the following reasons:
- Syntax errors: The email message or SMTP command sent by the sender does not adhere to the required syntax, making it impossible for the server to interpret or process the message.
- Invalid recipient address: The recipient's email address provided in the "RCPT TO" command or the "To:" field of the email message is incorrect or improperly formatted.
- Missing or malformed headers: Critical headers, such as "From," "To," "Subject," or "Date," are missing or incorrectly structured within the email message.
- Spam or security checks: Some mail servers may use SMTP error 501 as a way to reject emails that trigger spam or security filters due to suspicious content.
How to fix SMTP Error?
- Review the email message: Carefully review the email message content, including headers, subject lines, and recipient addresses, to ensure they adhere to standard email format requirements.
- Check recipient addresses: Verify that the recipient's email address is spelled correctly and formatted properly.
- Address syntax errors: Correct any syntax errors in the email message or SMTP commands. Ensure that commas, periods, and other punctuation are correctly placed.
- Avoid suspicious content: If you suspect spam or security filters are causing the SMTP error 501, review the email's content for potentially problematic elements, such as suspicious links or attachments.
SMTP Error Examples
Example 1: "501 Syntax error in parameters or arguments."
Example 2: "501 5.1.7 Invalid address in 'RCPT TO' command: <recipient@example..com>"
Example 3: "501 5.5.4 Invalid header format in email message."
Example 4: "501 5.7.1 Message content flagged as spam. Refused delivery."