Simply go to Arc > Settings > General > Privacy and Security, but make sure you select the correct Profile in your settings before doing this!
Once there, you’ll click "Site Settings," scroll down to the “Permissions” section, click “Additional Permissions,” and then select “Protocol handlers.”
From there, you’ll make sure that under the “email” heading you see your preferred email client.
If you don’t see your preferred email client listed, you’ll want to remove whatever is there by clicking the three dots on the far right side of it and selecting “remove.”
After that, you’ll simply navigate to your preferred email client within Arc, refresh the tab if it's a Favorite or Pinned tab, and select “Allow” when you’re asked if you want to make it your default.