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What do I use for email? When I'm not in a terminal window, I use Sylpheed:
https://sylpheed.sraoss.jp/en/
It's light, it's fast, it has keyboard shortcuts for common tasks, it resists do-everything feature creep, & its UI hardly ever changes between versions.
But it doesn't render received HTML mail except as very simplified text, & it won't let you send HTML mail except as a file attachment to plain text mail. This is not a bug, todo, or fixme. This is an incredibly useful & welcome feature.
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.