Outlook's view source opens the HTML as a .txt file.

You need to change the Windows Defualt App for file type .txt to Notepad++

Please note that this might also cause any other operation that opens a .txt file to open it in Notepad++

Outlook's view source opens the HTML as a .txt file.

You need to change the Windows Default App for file type .txt to Notepad++

Please note that this might also cause any other operation that opens a .txt file to open it in Notepad++