When I _am_ in a terminal window, I use Mutt, & for almost precisely the same reasons as Sylpheed.
http://www.mutt.org/
The biggest differences are that Mutt's founding inspiration is ELM, not Outlook Express, & it doesn't even pretend to render HTML mail at all, instead presenting them as file attachments to plain text messages. Mutt is also more extensible, more customizable, & fully keyboard driven.