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](https://superuser.com/questions/374139/how-to-change-the-default-program-used-by-outlook-2010-for-displaying-the-source) is more than  14 years old and should be valid for Outlok 2010, the OP mentions the solution doesn't work and I couldn't apply the solution either.

![enter image description here](https://i.sstatic.net/GAyryuQE.png)