Library /sys$common/syshlp/mailhelp.hlb REPLY, Qualifiers, /EDIT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /EDIT
       /NOEDIT

    Invokes a text editor to edit the reply you are sending. Unless
    you have selected a different editor, the DECTPU-based EVE editor
    is invoked. When you exit from the editor, the edited message is
    sent. To cancel sending a message, enter the appropriate command
    to quit from your selected editor. If you enter the DCL command
    MAIL/EDIT=(REPLY) and then decide that you do not want to invoke
    the editor for your response, enter the command REPLY/NOEDIT.