I started the 'eml-client' in github.com/OJFord/amail as a way of viewing them rendered (they are plaintext files, so one answer is you can just read in less or

I started the 'eml-client' in github.com/OJFord/amail as a way of viewing them rendered (they are plaintext files, so one answer is you can just read in less or whatever, depending what you need to see). However fairly early on I discovered notmuch, so then switched it from being a viewer for just a bunch of eml on disk to a GUI notmuch client. So if you want to use a recent version of it you'd need also to install notmuch (it's very lightweight) and create its index pointing to your eml directory. It's very much a slow WIP, afaik I'm its only user, no releases you'd have to build from source, etc. - but happy to help if you're interested.