Thanks to the ample Gentoo documentation, I have replaced ssmtp (with no Authenticated SMTP support) with msmtp (at least as far as mutt is concerned), which worked on the first try. Good thing because Earthlink is phasing out the in-the-clear (non-authenticated) SMTP servers, according to the Support guy I spoke with on the weekend.
Here is the exact article that introduced me to msmtp and which provided all the information I needed to set it up.