The way of how e-mail is handled on BeOS/Haiku is one of greatest things ever, I think. It allow to separate e-mail notification/fetching and e-mail reader application. So you do not need to run heavy memory consuming e-mail client with HTML rendering engine all the time. Mail being regular files allow to easily archive/backup or use different e-mail viewer software.
But another problem is that it currently do not work well. Most modern free e-mail services now do not allow access using login and password pair and require login using web page (OAuth etc.) that is currently not supported on Haiku.