The default program for "View Source" has been set to the Windows notepad by accident, Notepad++ should be instead the default program. How can be changed/reset this setting - see the below screenshot.
Apart from the fact that this post is more than 14 years old and should be valid for Outlook 2010, the OP mentions the solution doesn't work and I couldn't apply the solution either.

Update:
- I checked the default application for
.htmland what is defined in the Outlook section underWindows Settings > Apps > Default Apps > Outlook (or Outlook classic)) > - I had a look in the registry in the Outlook tree but I couldn't really find anything.
.htmlfiles? On searching have seen some suggestions that Outlook 2024 uses the Windows default application to view the HTML source of applications. This is a comment, and not an answer, since don't have Outlook 2024 to verify that changing the Windows default file association for.htmlwill allow you to use Notepad++.