set continuous-check

SET CONTINUOUS-CHECK NO/yes  -  whether to check for new mail at all times

With the usual NO setting, a check for new mail is done only at Top-Level
  (the MM> prompt), and not in Read or Send modes.  The check is done after
  each Top-Level command, and, when idle at Top Level, at the interval set 
  by CHECK-INTERVAL.

The purpose of setting CONTINUOUS-CHECK to NO is to avoid interrupting 
  reading and sending with the announcement about new mail and the pause 
  while it is saved.

If you change it to YES, the check is done in READ and SEND MODES just as
  at Top-Level, that is, after each command and, when idle, at the interval
  set by CHECK-INTERVAL.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   check   set check-interval