[jackalope]

Mutt has had excellent built-in SMTP/IMAP/POP3 support for years. I'm always surprised to see guides suggesting helpers like msmtp, offlineimap, etc. And with a few macros and save-hooks that you'll only write once, you'll have a fast, flexible mail client that will beat nearly any GUI client in mail processing speed.

[floatboth]

msmtp supports reading passwords from OS X's and Gnome's keychain programs. Also, having a separate program for that is very UNIX-way :)