Because of this problem with my keyboard’s mail button, I uninstalled Outlook — yet the problem remained, only that now instead of Thunderbird and Outlook the mail button opens Thunderbird along with the mail client of Opera!

I don’t know why this button should open mail programs in pairs and not all of them, but now I realized that in the registry Opera, Outlook and Thunderbird have some settings that need to be cleared, no matter what the Control Panel “Defaults” said. If the Control Panel settings were enough to define a program as exclusively “default”, the mail button would have been running just Thunderbird.

This is the main Registry key you need to visit, in order to remove whatever mail programs you wouldn’t like to use, if you wish your keyboard’s mail button opened just one program and not some strange pairs:

My Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail

In case that the problem persists, visit also:

HKEY_LOCAL_MACHINE\SOFTWARE\Classes\mailto\shell\open\command

HKEY_CURRENT_USER\Software\Classes\mailto\shell\open\command

HKEY_CURRENT_USER\Software\Clients\Mail

and

HKEY_CLASSES_ROOT\mailto\shell\open\command

What a beautiful day! Now my mail button opens just Thunderbird!