I'm using Fastmail and its web interface as my email client and I'd like to clean it up but save multiple hundreds of emails to some text-based format.
Is there any macOS email client or utility that can do it?
You can use Mozilla Thunderbird, with ImportExportTools plugin.
If you need a step by step guide, here is one with images, is quite old, but gives the idea. Use software/plugin from Mozilla site.