Either there is no default mail client or the current mail client cannot fulfill the messaging request - this is the message I get. cannot mail. Similar message for excel

Good Morning keithb,

Let's make sure this error message gets taken care of.

This error message appears when you try to send an email from a third-party program because the Outlook registry key is corrupted, and they cannot retrieve the required information from the registry.

To resolve this issue, you'd need first to remove the corrupted registry key, and then perform a repair. Please note that this process should be done by an IT professional or your email provider as fixing your computer's registry, if performed incorrectly, can severely damage the operating system and render the computer unusable.

You are always welcome to check with us if you need help about QuickBooks. We're ready and excited to help you, keithb.

The last step that worked for me was to Run the office 365 setup for current Windows user profile. 
Office / outlook was already installed on this machine for more than a year and current user was using it n never had any issue. I tried run repair but that did not make any difference. But running office 365 setup again resolved the issue. Not sure if that was something with registry.

Previously I tried changing QB compatibility setting, resetting Default email client as Outlook, running QB + MS office + Windows update, Repair IMAP,l.

I tested sending email from QuickBooks after each step.