Hi, Leo1.
I can share some information regarding the email headers in QuickBooks Online.
Changing the email header is currently unavailable in QuickBooks Online. In the meantime, I recommend visiting the QuickBooks Blog website to stay up to date on any updates or improvements on the online version.
You can bookmark the following link: https://quickbooks.intuit.com/uk/blog/.
In case you need help with utilizing your sales form templates in QuickBooks Online as well as other information you can add, you can go through these resources below:
Customize invoices, estimates, and sales receipts in QuickBooks Online
Importing Custom Invoice Templates into QuickBooks Online
Feel free to drop me a reply below if you have any other concerns. I'd be more than willing to lend you a hand.