Mailnex is an email client that is currently a rough clone of heirloom-mailx, which (as of 2016) appears not to be maintained any more. The end goal is to have a MUA capable of IMAP that has a CLI like mailx, but with some modern features like line editing, history, and tab-completion. It also should provide indexed searching and decent thread browsing (similar in that way to the 'sup' mail client)
Source Code and Project Page are on github.
The project is currently progressing faster than I can keep this website up-to-date, so for now check the github page for progress, which should hopefully be hosting a more up-to-date "README" file.