riot is almost a Haskell mail client

In case anybody wanted to write a mail client in Haskell, I should point out that Tuomov's riot (Riot is an Information Organisation Tool) outliner

• provides a sort of mutt-like user interface and
  
• stores its outlines as mailboxes (using in-reply-to to treat outline ancestry as thread ancestry).

So that's some of the work done for you :-)