With Office 365, the default mail client is not set to Outlook. It sounds like a simple issue, but after a long search for a solution, it ends with a surprise! All starts with using the Adobe Acrobat Reader function to send a PDF document through the default email client. However, the use ends with an error message that no default email client is available. The default email client is defined under the registry key HKLM\SOFTWARE\Clients\Mail, but precisely that key is deleted when ending Outlook!

The first question: Is Outlook responsible for deleting the key? That's the reason for using Microsoft procmon to find that out. Procmon reveals that Outlook is not accountable, but the process OfficeClickToRun.exe is. So, what's the use of the process, and that is not fully clear. "Installing and updating" Office, and often, there is a comment for something else that is not explained. Anyway, an Office component, and maybe there is a permission or policy responsible for it, but it could be out-ruled quickly. A test on the Master Target reveals that the issue could not be reproduced. Now, a famous question: What is the difference between Master Target and a productive image? With that question, another component came into the game: The FSLogix rules for Microsoft Office.
The users had permission by FSLogix rules, but the system account didn't, and the Microsoft Office Click-to-Run service uses the system account. Once the system account was added, the service could access the user licence.

FSLogix Rules

Still, the question is unanswered why the Office Click-to-Run service removes the default email client key? If Outlook or any other Office program wouldn't start because there is no access to the user licence, that would be understandable. Why just deleting the key? That’s a riddle that only Microsoft could reveal.