For my personal convenience, years ago I decided to use GMail as my mail client, meaning that all my email addresses send their email to my GMail account and, using IMAP, I can send email from the GMail panel. I formatted my mac, installed High Sierra and opened the email client to setup as default email application the browser and not. Guess what, I cannot access 'preferences' unless I don't setup and email address (which, honestly, I'm not willing to do). So, in the mind of Apple, to change the default client email I have first to use it. Is there a way to avoid this?
Currently, macOS High Sierra is available in the Mac App Store, but it will only run on certain Mac computers. Interested users with a compatible machine can download the OS today.