550 the mail server does not recognize as a valid sender

The infamous "550 the mail server does not recognize as a valid sender" error!

This error typically occurs when the mail server receiving the email does not recognize the sender's email address as a valid or authorized sender. Here are some common reasons that might cause this issue:

  1. Invalid or non-existent email address: The sender's email address might be misspelled, invalid, or non-existent.
  2. Domain not authorized: The sender's domain might not be authorized to send emails through the receiving mail server.
  3. SPF (Sender Policy Framework) issues: The sender's domain might not have a valid SPF record, or the record might not be correctly configured, which can cause the receiving mail server to reject the email.
  4. DKIM (DomainKeys Identified Mail) issues: The sender's domain might not have a valid DKIM record, or the record might not be correctly configured, which can cause the receiving mail server to reject the email.
  5. Blacklisting: The sender's IP address or domain might be blacklisted, which can cause the receiving mail server to reject the email.
  6. Authentication issues: The sender's email account might not be properly authenticated, which can cause the receiving mail server to reject the email.

To resolve this issue, you can try the following:

  1. Verify the sender's email address: Ensure that the sender's email address is valid and exists.
  2. Check the sender's domain: Verify that the sender's domain is authorized to send emails through the receiving mail server.
  3. Configure SPF and DKIM: Ensure that the sender's domain has a valid SPF and DKIM record, and that they are correctly configured.
  4. Check for blacklisting: Verify that the sender's IP address or domain is not blacklisted.
  5. Authenticate the sender's email account: Ensure that the sender's email account is properly authenticated.
  6. Contact the mail server administrator: If none of the above steps resolve the issue, contact the mail server administrator to investigate further.

Remember that the specific solution will depend on the root cause of the issue, so be sure to investigate and troubleshoot accordingly.