Email is the emperor of asynchronous text-based communication. The formats are well defined and tiny, and adoption is very high.
mailcap
Mailcap specifies which extensions to use to open files with. Out of the box
mutt performs horribly with anything but plain-text. Here's an example
.mailcap file:
text/plain; cat %s; copiousoutput
text/html; links -dump %s; nametemplate=%s.html; copiousoutput
application/pdf; /usr/bin/evince %s
text/pdf; /usr/bin/evince %s
image/*; feh %s
mutt
Secure mail
Probably the best way of securing email is to host your own provider. After the lavabit shutdown, it's probably a good option to use riseup.