Even though it was first released in 1995, Mutt still enjoys an active community following it and having new releases constantly.

Allows to filter (called "limit") mailbox by various patterns using regular expressions. Filtering also can be used to mass-tag mails and apply certain operations on them, like deleting or saving.

Supports maildir and mbox, but also others like MH and MMDF.

Colors and keybindings can be fully customized.

Has support for integration of PGP, GnuPG and S/MIME.

View can be switched to threaded mode to display replies under initial mail. Allows common operations on threads, like delete, mark as read, tag, collapse/expand or delete.

Can be run remotely.

Quick startup which allows it to be run only when needed.

Uses less resources than graphical mail clients.