I've found this workaround works as well - frustrating to take th extra steps, but it does work for the moment.  Copied from another thread with the same issue:

We were able to find the following workaround for now that I hope will help anyone else having this issue:

1. Pull up the email like you normally would, from your invoice or sales order screen. Don't make any customizations as they won't flow through to Outlook, but SAVE it. (For my client there was a button for this at the top of the email window.) This actually saves it as a draft in your Outlook.

2. Close the email from within Quickbooks. It is locked from editing in Outlook as long as it's open in Quickbooks.

3. Go to your drafts folder in OUTLOOK. You can open the email here and make any customizations you need to and send to the customer.

My client was able to do this and send their customized messages still. It's an extra step, but will let you do what you need to until this issue is fixed by Quickbooks.