It may be that your computer still has the default 'MAILTO' protocol setting as Thunderbird.

Control Panel > Default Programs select 'Associate a file type or protocol with a program' Under 'Protocols' select: MAILTO line click on 'Change Program' Select the one you want as default' click on 'Close'

The MAILTO Protocol was set correctly to Outlook 2016, and Outlook 2016 was set as the default email program as well. It still calls up Thunderbird. The only way to have it correctly call up Outlook is to uninstall Thunderbird.

Thunderbird is not setting the Mailto, that option has to be selected so that the computer or program knows what to use.

Test to see if the default mail client keeps changing depending upon which Email Client was last opened.

Please close Outlook if open and then open Thunderbird. Check your computer 'MAILTO' settings to see if they have changed.

In Thunderbird, please check this setting:

• 'Menu icon' > 'Options' > 'Options' 'Advanced' > 'General' tab

under 'system Integration' section

• uncheck 'Always check to see if Thunderbird is the default mail client on startup'
• click on 'OK'

If you use eg: Firefox browser program - check the 'MailTO' settings: open Mozilla Firefox browser 'Tools' > 'Options' > 'Applications' 'Mailto' choose the default program you want to use.

re :many programs when I use that program's "Send as email" function it brings out a Thunderbird

You need to find out what you have got setup for those programs. As an example....if you open Adobe Reader open any pdf document File > Preferences > Email Accounts check the settings.

It doesn't matter which program I use to send mail (Word, Excel, Paint, PdfXChange, etc., all of them will use Thunderbird if it is installed, regardless of the fact that Outlook is set as my default in both Windows and the individual programs. As far as the Thunderbird setting you mentioned, I have it set not to check to see if Thunderbird is the default. In Firefox, the default "mailto" shows "Use Outlook 2016 (default)", yet it will still use Thunderbird. I first saw this come up in using PdfXChange, will also is set to use Outlook but will call up Thunderbird anyway. I have spoken to them about it and they have been able to duplicate the problem and can't figure it out either. It has to be something that Thunderbird is doing.

Please check registry key: HKEY_CURRENT_USER\Software\Clients\Mail

'Start' > Run > type regedit Locate this registry entry: HKEY_CURRENT_USER\Software\Clients\Mail on right side double click on 'Default' Enter Microsoft Outlook in 'Value Data'

Windows 10 may have some variants. eg: HKEY_CURRENT_USER\Software\Clients\Mailto

also check: HKEY_CLASSES_ROOT\mailto\shell\open\command

I'm not using Windows 10. Suggest you use google to double check. such as : https://www.windows10forums.com/threads/send-to-option-not-recognizing-mail-as-default-email-program-even-when-set-as-such.6725/ treborkalarg comment. https://stackoverflow.com/questions/3940358/how-and-where-exactly-does-the-default-email-client-gets-set-in-the-registry

Thank you, Toad-Hall. That registry change did the trick. I am using Windows 10, but the registry key was still "...\Clients\Mail" (I didn't see any "...\Clients\Mailto" key). That key was still showing Thunderbird, even though I had uninstalled Thunderbird. For some reason that key wasn't getting updated.

Thanks so much!

Ken