Using New Outlook on Windows for personal email, calendar, and contact management
Dear Greg Brown2,
Good day. After thorough research, test from my end and consulting, we find out that: when we type "sand_wich_" in new outlook, the word is highlighted as an error and the new outlook give you some suggestions to correct the word but it doesn't automatically correct the word.So if you don't want the corrected spelling you can disable it , new email "Options --> Editor > Editor setting and disable all the correction option and disable auto correct as well"
However in new outlook , it is not currently feasible to disable "*Bold* and _italic_ with real formatting" because it work as Outlook on the web.
Since this product is in its Preview phase, customers may experience some gaps in functionality. Currently the only support offered for this product is in-app support that is in direct connection with Product Group. That said, customer need to create an in-app ticket; please refer to Contact support and provide feedback in new Outlook for Windows - Microsoft Support.
Additionally, it is advisable to verify the functionality in the old/legacy Outlook, and if it operates correctly, consider using the old Outlook as an alternative.
We apologize if we are not able to meet your requirements; however, thank you for bringing this to our attention, we will surely raise your concerns to our Product Team and hopefully this can be included in the future updates.
However, try to disable the spelling check in new outlook and check the issue as mentioned above and also make sure is happen only in new outlook by check in word app as well.
We look forward to your reply. This requested information will help us narrow down the scenario and find the right troubleshooting. We will continue to assist you based on the information you provide. I sincerely appreciate your patience and cooperation.SincerelyEben Ezer Tres | Microsoft Community Moderator