Most of the usual Emacs commands are available in Rmail mode, though a few, such as C-M-n and C-M-h, are redefined by Rmail for other purposes. However, the Rmail buffer is normally read only, and most of the letters are redefined as Rmail commands. If you want to edit the text of a message, you must use the Rmail command e.
Edit the current message as ordinary text.
The e command (rmail-edit-current-message) switches from Rmail mode into Rmail Edit mode, another major mode which is nearly the same as Text mode. The mode line indicates this change.
In Rmail Edit mode, letters insert themselves as usual and the Rmail commands are not available. When you are finished editing the message and are ready to go back to Rmail, type C-c C-c, which switches back to Rmail mode. Alternatively, you can return to Rmail mode but cancel all the editing that you have done, by typing C-c C-].
Entering Rmail Edit mode runs the hook text-mode-hook; then it runs the hook rmail-edit-mode-hook (Section 32.2.3). It adds the attribute edited to the message. It also displays the full headers of the message, so that you can edit the headers as well as the body of the message, and your changes in the headers will be permanent.