I have Office 2008 and the default application for .eml files is Entourage. In Office 2011, Outlook replaces Entourage; perhaps Outlook will work for you. Also, Mail is listed as a possible application for those files. BTW, I'm using RCDefaultApp.

If you do a Get Info on a .eml file (or control-click it and select Open With), what does it say?

Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365