550 invalid syntax in mail command

A classic error!

The "550 invalid syntax" error typically occurs when the mail server rejects the email due to incorrect or malformed email syntax. Here are some common causes and solutions:

  1. Invalid recipient email address: Check that the recipient's email address is correct and formatted correctly (e.g., [email protected]).
  2. Missing or incorrect headers: Ensure that the email has the required headers, such as From, To, Subject, and Date. Verify that the headers are properly formatted and not missing any essential information.
  3. Invalid or missing body: Make sure the email body is not empty and contains valid text or HTML content.
  4. Incorrect or missing MIME type: Verify that the email has a valid MIME type (e.g., text/plain or text/html) and that it's correctly specified in the Content-Type header.
  5. Excessive or malformed attachments: Check that attachments are not too large (exceeding the server's limits) and that they're properly formatted and encoded.
  6. Invalid or missing authentication: Ensure that you're properly authenticated with the mail server using the correct username and password.
  7. Firewall or network issues: Verify that your network connection and firewall settings are not blocking the email transmission.

To troubleshoot the issue, you can try the following:

  1. Check the email syntax using a tool like swaks (a command-line email client) or telnet to connect to the mail server and send a test email.
  2. Verify the email headers and body using a tool like mail or mutt to inspect the email contents.
  3. Check the mail server logs for any error messages or warnings related to the email transmission.
  4. Consult the mail server documentation or contact the server administrator for specific guidance on resolving the issue.

If you're still stuck, please provide more details about your email client, mail server, and the exact error message you're seeing, and I'll do my best to help you troubleshoot the issue!