_ Balsa _ is available in Debian-based distros. It is super lightweight. It sports a slightly retro aesthetic.

https://pawsa.fedorapeople.org/balsa/

Then there is Claws-Mail, which is under heavy active development with a community of users, patchers and bug reporters staying on top of it like white on rice. It is also very light-weight compared to Thunderbird.

I like the Thunderbird interface. I hate the Thunderbird configuration and storage backends. Claws uses good old MH and so does Balsa.

If you choose Claws it also has NNTP reading built in. Balsa has no NNTP functionality.