There are ways how to export customer contacts and emails. I'll show you how to do it in QuickBooks Online.
Here's how you can export a list of your customer's email addresses by heading to the Reports tab:
On the left panel, click Reports.
Type Customer Contact List in the search box.
Click the Customize button located in the upper right-hand corner.
Under Rows/Columns, click the Change Columns link.
Put a checkmark on the Customer and Email box.
Click Run Report.
Click the Export icon to PDF, Excel, next to print and manipulate them there.
By clicking the Customize button, you can also modify the report. If you only need the customer's name and email address, uncheck the unnecessary details in the Rows/Columns section.
I've attached this sample report for your reference:
Another way is to export customer details from the Sales tab.
Here's how:
Go to Sales, and click the Customers tab.
From there, choose the Export icon beside the Print icon.
Then, hit the Export to Excel box.
Click Enable Editing to manually remove other data within the excel file.