Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
I had the same issue and found that you could delete emails properly using the delete key on the keyboard as other have found.
I managed to fix the issue by:
Open Outlook, File > Options > Customize Ribbon.
In the right hand pane, remove the “Delete” button from the Home (Mail) tab.
Close Outlook and re-open Outlook.
File > Options > Customize Ribbon.
In the right hand pane, tick the Home (Mail) tab and remove tick for all other tabs
Click the Reset button drop-down menu
Select "Reset only the selected Ribbon tab"
This will add the Delete button back to the ribbon.
Now when you delete a email using the Delete button it should work properly.
This fix worked for me and may not necessarily work for others.