SSMTP:

The first is ``ssmtp'' (``simple SMTP''),a package that allows you to send mail via the SMTP protocol to your ISP, authenticating with a username and password. For most people, that's all you need to do anyway, and choosing this option simplifies the process of installing mail transport agents that were really intended to go on servers and handle immense volumes of mail, spam, and bounces every day.

If that's the mechanism you choose, your .muttrc file should look like this:

set sendmail="/usr/sbin/ssmtp -audUserName@domain -apSecretPassword"