set control-n-abort

SET CONTROL-N-ABORT ASK/always/never  -  whether control-n aborts sending

With the usual ASK setting, when you type control-n during the send 
  procedure, MM asks whether to abort.  Control-n can be used to abort
  at the "To", "cc" or "Subject" prompts and while typing the text of
  the message.

If you change it to ALWAYS (or YES), control-n will abort immediately;
  that saves one command, but makes it possible to abort by mistake.  (If
  you do abort by mistake, go back by using CONTINUE at the Send> prompt.)

If you change it to NEVER (or NO), control-n will do nothing, so you
  cannot abort.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show
(at the MM> or Read> prompts:)  send