Scenario
Even though Outlook is set as the default email client in Windows, when you use that program’s “Send as email” function it brings out a Thunderbird dialog box instead of Outlook.
Solution
Open regedit. (‘Start’ > Run > type regedit)
Check registry key: HKEY_CURRENT_USER\Software\Clients\Mail
right side double click on ‘Default’ Enter Microsoft Outlook in ‘Value Data’
Windows 10 may have some variants
- HKEY_CURRENT_USER\Software\Clients\Mailto
- HKEY_CLASSES_ROOT\mailto\shell\open\command
