This mail has been blocked because the sender is unauthenticated

The infamous "unauthenticated sender" issue!

When an email server receives an email, it checks the sender's authentication to verify that the email is legitimate and not spam. If the sender's authentication fails, the email server may block the email to prevent spam and phishing attacks.

Here are some common reasons why an email sender might be considered "unauthenticated":

  1. No SPF (Sender Policy Framework) record: SPF is a DNS record that specifies which IP addresses are authorized to send emails on behalf of a domain. If the sender's domain doesn't have an SPF record or the record doesn't match the IP address of the sender, the email server may block the email.
  2. No DKIM (DomainKeys Identified Mail) signature: DKIM is a digital signature that verifies the authenticity of an email. If the sender's email doesn't have a valid DKIM signature, the email server may block the email.
  3. No authentication headers: Some email servers require specific authentication headers, such as "Authentication-Results" or "X-MS-Exchange-Organization-AuthResult", to be present in the email. If these headers are missing, the email server may block the email.
  4. Invalid or missing sender email address: If the sender's email address is invalid or missing, the email server may block the email.

To resolve this issue, you can try the following:

  1. Check your domain's DNS records: Ensure that your domain has a valid SPF record and DKIM signature. You can use online tools like SPF Record Checker or DKIM Checker to verify your records.
  2. Configure your email server: If you're using a mail server like Postfix or Sendmail, you may need to configure it to include authentication headers and signatures in your emails.
  3. Use a mail client with authentication: If you're using a mail client like Microsoft Outlook or Mozilla Thunderbird, ensure that it's configured to use authentication (e.g., SMTP authentication) when sending emails.
  4. Contact your email provider: If you're using a free email service like Gmail or Yahoo, contact their support team to see if they have any specific requirements for sender authentication.

By resolving these issues, you should be able to send emails without being blocked due to unauthenticated sender concerns.