Please note that we have been seeing the exact same problem since applying the last major update to Quickbooks. We have the latest version of QB Premier Plus Edition 2021 as of August 26.

Outloook version 2010 Build 13328.20292

The only workaround solution I have found is to first change the message format to Plain Text, then change the email format back to HTML, before sending the email.

This needs to get fixed!!