Thanks for joining this thread, @RachieB. 

To clarify, are you trying to change your clients' email addresses from your account as well? Once your clients make some changes to their account, it will automatically update their information in your QuickBooks Online Accountant. 

 address that appears on sales forms:

• Change your customer-facing email address
• Change the email address your customers see

Feel free to post here in the Community if you have any concerns about emailing invoices in QuickBooks. Have a good one.