I appreciate you following up with the Community, coffeelady1.
 

Our Product Investigations team conducted an investigation (INV-42598) and determined that the "Outlook cannot log on." message is caused by an outdated Microsoft Office 365 build. This error can be resolved by updating to the program's latest release.
 

Here's how:

1. Open Outlook.
2. Access your File tab.
3. Choose Office Account, then click the About Outlook button.
4. Hit Update Options.
5. Select Update Now.

Now that you're using the latest release of Microsoft Office 365, you can return to your books and send emails as normal.
 

You'll additionally be able to find many detailed resources about using QuickBooks in our help article archives.
 

I'll be here to assist if there's any questions. Have an awesome day!