Implement SASL PLAIN authentication in SMTP server
The previous method of handling authentication was replaced with SASL PLAIN mechanism, allowing to standardize and secure the authentication process. Also, the error response for unsupported authentication mechanisms is added. Lastly, some settings for the fake SMTP server are put in place, including enabling Delivery Status Notification (DSN).
Signed-off-by: Christian Roessner c@roessner.co