README:

For smtp-auth to work, you need to install a sasl plugin.
(libsasl2-plug-plain and libsasl2-plug-login for the default setup)

To find more information on STARTTLS and AUTH, see:

http://www.sendmail.org/~ca/email/starttls.html
http://www.sendmail.org/~ca/email/auth.html
https://www.TBS-internet.com/ssl/sendmail-tls.html