Usually, sometimes have to open email and export as “raw message source”. Mail Raider.app will view these, but Spotlight doesn’t index contents. System viewer will show correctly. EML is RFC 822 standard (1982!), best description of current state is a .gov document. I doubt Mail.app is fully compliant, particularly with images.

Windows Mail.app will open these files, but doesn’t display inline images.

I archive key emails in project folders this way, share via Google Drive. Old feature, little known. Not the worst archival file format and easy to do and share.