Use an account and destination you are authorized to test.

  1. Resolve the submission hostname.
  2. Connect on the documented port.
  3. Inspect the greeting and EHLO capabilities.
  4. Require TLS according to the service design.
  5. Validate the certificate.
  6. Authenticate using the supported mechanism.
  7. Submit a controlled message.
  8. Preserve the server replies and delivered headers.

After STARTTLS, issue EHLO again because advertised capabilities can change.

Never paste production passwords or bearer tokens into public testing websites.

Reference material