SET REPLY-ALL NO/yes  -  whether to use the option "all" in replies

With the usual NO setting, the command REPLY assumes the reply should
  go to the sender of the original message.  You can specify the option
  "all" when you want to send the reply to everyone in the TO and CC
  fields of the original.

If you change it to YES, the "all" option is then assumed, and you 
  would have to specify the "sender" option to REPLY only to the sender.

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