SMTP authentication logs: capture the stage without exposing credentials

Define the decision

An SMTP log can contain sensitive authentication material, even when it looks like base64 text. Share only the server, port, TLS mode, stage, response and timestamp needed to diagnose the failure.

Work through the procedure

1. Identify whether the connection reached DNS, TCP, TLS, authentication or message submission.
2. Keep the full provider response but remove credential payloads and tokens.
3. Compare the failing configuration with current provider instructions.
4. Route the failure to the network, account-policy or application owner indicated by the stage.

Worked example

The following is a synthetic example for this procedure, not a customer result or performance benchmark.

Sanitized record:
Host: smtp.provider.example
Port: 587; transport: STARTTLS
TLS: established
AUTH: rejected with provider explanation
Credentials and AUTH payload: intentionally omitted

Read the result

The stage sequence narrows the cause. If TLS never completed, a password reset is unlikely to address the first failure. If AUTH succeeded, investigate the later sender or submission response.

Check before moving on

1. Check logs before attaching them to a ticket.
2. Preserve the account identity in an appropriately restricted support channel.

Limits and next action

Do not enable verbose authentication logging in production without a redaction and retention plan. Never ask another user to paste an AUTH command payload.

Source: RFC 5321: SMTP transaction stages