Claws Mail [1]. I used it since when it was called Sylpheed (later Sylpheed Claws), and wouldn't swap it with anything else.
Features aside, it's rock solid, small and fast, truly fast: I can search my entire mail db since like 1997 and find an indexed word in a sender/recipient/subject in a fraction of second, or a word in the body in more time.
The site hasn't been upgraded for a while and looks dated, but the software itself is very actively maintained. Currently using 3.17.4, which was released at the end of July.
There is also a port to Windows, which I used in a place where Outlook was having problems, after importing all its data (through an FOSS utility called "readpst"). The operator dropped his jaw looking at the difference in speed as apparently long searches in the message base were among his duties.
[1] https://www.claws-mail.org/
The site hasn't been upgraded for a while and looks dated, but the software itself is very actively maintained. Currently using 3.17.4, which was released at the end of July. There is also a port to Windows, which I used in a place where Outlook was having problems, after importing all its data (through an FOSS utility called "readpst"). The operator dropped his jaw looking at the difference in speed as apparently long searches in the message base were among his duties.