Using Classic Outlook on Windows for personal email, calendar, and contact management
For me this is caused by the mouse wheel being fired which causes the window focus to change to Message and not the Inbox anymore, which presumably then confuses the keyboard handling routine as it thinks the menu has focus now and not the text input control and Del means delete message not the text.
Clicking in the message input control does nothing to fix this focus shift to Message (Im not sure why) and you need to click on the window scroll bar or somewhere in the main Outlook window or title bar to bring focus back to the edit window again.
I have tried to fix it for me using AutoHotkey but I cant quite get it right yet.