To all other users, please see my previous post (on page 1 of this thread) if you are still having difficulty with the alias workaround.
I have also built a separate thread to provide some background on the reason for which this issue arose, why these changes were made, and the different options available to users to resolve it: https://community.hubspot.com/t5/CRM/Sending-emails-from-the-CRM-recent-changes-prompting-This-email/td-p/13974
May