set reply-indent

SET REPLY-INDENT "[string]"  -  define the marking for inclusions in replies
Usually:         "> "

The REPLY-INDENT variable defines a string to be used to set off the 
  included message when including the original message into REPLY.
  The usual string is a > and a blank space.  You must use quotation 
  marks around the string.  

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