You can also use Opera Mail that's built into the old Presto-based Opera Browser suite. You can download it from https://ftp.opera.com/pub/opera/win/1218/int/.
Just like the standalone Opera mail though, there are issues. There's no TLS 1.3 support, there's no oAuth authentication support and certificate and cypher support is very outdated. The standalone Opera Mail also has some crash issues in certain cases. The rendering of HTML messages is also outdated due to Opera Mail using an old rendering engine.
Besides those things, if you can workaround them, you can still use it.
Note that Vivaldi now has a built-in mail client that attempts to replace Opera Mail. It's pretty decent and modern.
