Kmail UI bug: explain the choice of authentication methods!

when I configure kmail > accounts > modify/add > security > check what the server supports, dreamhost and ouvaton only support 'clear text' and 'plain' authentication methods. My server (lesenjeux.fr) supports everything! Where is this set on the server? What's the difference between the different authentication methods? By the way, this is another way where linux/kmail is not so user friendly: it offers plenty of choices but does not explain the differences!