I have a couple of solutions we can do to help you change the email provider to Outlook.
When you hit the Email button, there will be an "open with" window that will pop-up where you can select an email provider. Did that appear in your end? If not, you'll want to enable the pop-ups in your browser.
You can also try sending an email through a private or incognito browser. This might have something to do with the cache or the cookies (website user preferences). Here are the shortcut keys:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Internet Explorer: Ctrl + Shift + P
Safari: Command + Shift + N
When you've successfully set Outlook as the default mail provider, go back to your regular browser and clear its cache and cookies. Close your browser afterwards to complete the process.
When you're done, you can check out these topics to help you manage your sales and customers in QuickBooks Online: Sales and customers.
I'm more than happy to help you if you have follow-up questions within the Customers section. I also welcome any other future questions you might have for QuickBooks Online.
Changing the default email provider in QuickBooks from Outlook to Gmail is unavailable. For now, we may need to manually open and send emails using the email platform you wanted to use.
Saying you wanted to maximize the use of this feature, I recommend letting your voice be heard throughout the entire platform by sending your experience via feedback. I'll walk you through the steps to accomplish this.
Go to the Gear icon.
Under PROFILE, select Feedback.
Describe how you wish to change the email provider preference and how it is helping you work efficiently in QuickBooks.
Select Next.
Click Skip and send message.
I'm including our Customer Feedback for QuickBooks Online page to track the status of your request and get the latest updates from our developers. Updates such as how the recent enhancement made to QBO helps our business owners, like you.
It'll always be my pleasure to help if you've got other questions aside from working with your company settings in QuickBooks. I'll be here to help.
Have you tried opening your QuickBooks using a private window (Incognito)? If not, let's go ahead and open your QuickBooks using incognito. It is possibly a browser-related issue. Also, this session disables the corrupted cache in your browser and prevents unexpected QuickBooks Online behavior.
Here's how:
Google Chrome: Press CTRL + Shift + N.
Safari: Press Command + Shift + N.
Mozilla Firefox: Press CTRL + Shift + P.
If it works, you may want to clear the browsing history of your regular web browser. This will remove previously stored browsing data that might have caused the issue. Otherwise, you can try using other supported browsers to be thorough. Another option is to use the browser health checkup tool to see if your browser is compatible.
If the same thing happens, I suggest contacting our support team directly. They have the tools to further investigate and check your account in a safe environment, and they can help find the root cause of your issue and find a fix.