Alpine
Used by 3 peoplewashington.edu ↗

“As the last representative of the dinosaur era, I read mail mostly with Alpine, which is faster and easier than any web-based system I know, and does a nice job of storing my mail locally.”
Brian Kernighan Interview, Computer scientist (Unix), writer · via Uses This 
“My parents are both computer scientists of the latter type (my dad checks his email with Alpine!), and I inherited it from them.”
Nozlee Samadzadeh Interview, Writer, editor, IA (Flat, The Morning News) · via Uses This 
“I'm thinking of switching to Alpine but I haven't made the leap yet.”
Andrew Plotkin Interview, Hacker, IF Wizard · via Uses This