set browse-pause

SET BROWSE-PAUSE YES/no  -  whether to pause during BROWSE

With the usual YES setting, MM will pause between messages when you are
  using the BROWSE command.  

If you change it to NO, there is no pause between the end of a message
  and the BROWSE prompt for the next message.  This will make it impossible
  to read the end of a message unless you also SET BROWSE-CLEAR-SCREEN NO.

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