Quickbooks....Outlook not default email program

• Double check that Outlook is truly set as the default under all Win 10 settings, it’s mailto protocol, and within Outlook itself — reference this if needed:

• How to Set Outlook as the Default Mail Client

• If a lot of changes haven’t been made to the station, system files etc. then you could try running a very recent System Restore point if ones available. 
• Otherwise, from what I can tell there is not a one solution fits all for this one. I would recommend a process of elimination for resolving this following their KB article — different error but same applies for troubleshooting. You have already checked for Office updates it appears:

https://quickbooks.intuit.com/learn-support/en-us/sync-applications/how-to-fix-error-quickbooks-is-unable-to-send-your-email-to/01/186062

Ok...
1. Outlook is default email client in W10....
2. No system restore points available...
3. Office is updated...
4. QB is updated...
5. Got QB Tools Hub...ran a couple diagnostics...no joy...
6. Unistalled QB....reboot
7. Reinstalled QB....opened data file....seems to be working ok....
8. Went to Preferences > Show Forms......and QB locks up...it will NOT show the System page where the radio button for setting Outlook as default email....
9.  In effect...absolutely no change from original....
10.  AND computer #2 still works correctly....Outlook IS the Default email and that comptuer can send out invoices....
BUT....the main computer does not....

What to do next...????

Jazz..we got it...but it was not easy....I brought in a tech buddy who is much more knowledgable in QB than I am...
We worked for 2 hours and of all the things we did we think it was a combination of 2 things....WIN.ini was missing MAPX=1 statement and somehow or another we decided that Office 365 was not communicating with QB so we did the full online repair....which is essentially a re-install.....that gave us a licensing problem that we had to work out....
But after that was fixed....then QB would recognize Outlook as the default email....tested and working....I think the link yo provided above was most helpful in "hinting" to the issue....Thank  you...

Glad it’s resolved! Very strange how that would suddenly change.

We don't have any idea what happened to create the issue....Previous to 11/3 everything was perfect for the past year....Why it worked without the MAPX=1  in the win.ini file makes not sense.....I had scanned for virus on the 4th and everything was clean....that's the fun thing about computers.....you never know....