Buried in a ribbon in every email message is a command to enable the editing of an email message. In Outlook 2019, it's here:

"Message" tab
"Actions" widget (which is about half way across the screen)
"Edit message" menu item

In other words, the command is in a location nobody would ever guess! For reasons I don't understand, the command is not available in every email message.

This sequence might get you there by pressing only four keys, one after another:

F10
h
a
e

No guarantee the "Edit message" command is in the same location, or can be reached via the same key sequence, in other versions of Outlook.