SMTP Error 501 occurs when there is a syntax error in parameters or arguments of the email message or SMTP commands. This error indicates a failure in processing the email due to incorrect formatting.

How to fix SMTP Error 501?

To resolve SMTP Error 501, follow these steps:

• 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 501 Examples

• Case 1: "501 Syntax error in parameters or arguments."
• Case 2: "501 5.1.7 Invalid address in 'RCPT TO' command: <recipient@example..com>"
• Case 3: "501 5.5.4 Invalid header format in email message."
• Case 4: "501 5.7.1 Message content flagged as spam. Refused delivery."