I retract the above for Windows 10 Version 1803. I have not seen this before, but I have Outlook installed, but not configured, and the Win10 Mail App is set as the default on this machine and is also not configured. I use webmail and never use the Send to->Mail Recipient option except when testing.
I am getting the same result as you do. It wants me to create an Outlook profile, and this is definitely a behavior that is not "as designed" since I've done this on earlier versions of Windows 10 and the default e-mail client was always invoked.
Definitely file a report via the Feedback Hub or Upvote an existing one for the same problem.
Edited by britechguy, 06 June 2018 - 10:29 AM.
Added note about using Feedback Hub to report
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
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