Hi there, @rosa_grijalva.
Thanks for checking us out here in the Community. I'd be happy to guide you in having your customers email transferred over to your default email in QuickBooks Online.
Are you trying to import customers list from Excel? If so, you'll need to make sure that the format on your Excel is correct for this might be causing the issue.
There are the following:
Once verified that the information is correct you can go ahead and import the customers' list by following these steps:
To know more about this process, please check out this article: Import customer, vendor, or supplier contacts from Outlook, Excel or Gmail.
Please let me know if you're referring to something else, I'll be around to provide additional assistance. Have a great day!