I have some insight about sending sales emails to a customer using our sales representative's emails using QuickBooks Online (QBO), and I'll share it with you, @GeorgeN.
After you email a customer, any responses will be sent to the company's customer-facing email by default. Please note that you cannot use multiple emails with QBO. However, you can use the Note to Customer section to leave a note for your customers and include your sales representative's email there. This way, the customer can respond to the sales representative's email address.
See the photo attached below as a visual guide:
In managing your sales forms, note that you can also personalize them to fit your business standards. To know more, see this attached link for a guide: Customize invoices, estimates, and sales receipts in QuickBooks Online.
With managing company emails or other questions with QBO, drop it down below. You can expect help from us anytime.