!msg nick(use _) msg to be sent
!yes
!no

*yeah it is pretty pointless but hey you never know :)

on *:text:!msg*:*:{ 
  if (%xmsg. [ $+ [ $2 ] ] == $null) {
    set -u60 %xmsg. [ $+ [ $2 ] ] $3-
    set -u60 %xnick. [ $+ [ $2 ] ] $2
    set -u60 %acceptpm. [ $+ [ $2 ] ] on
    set -u60 %senick. [ $+ [ $2 ] ] $nick
    .msg $2 10msg from4 $nick $+ 10, type 4!yes10 or 4!no10.
    .notice $nick 10msg to4 $2 10has been sent. Please wait for4 $2 10 to accept or decline 
  }
  else notice $nick 10Someone has already sent a message to:4 %xnick. [ $+ [ $2 ] ] $+ 10. Please wait
}
on *:text:!yes:?:{
  if (%acceptpm. [ $+ [ $nick ] ] == on) {
    .msg $nick 10Msg from4 %senick. [ $+ [ $nick ] ] $+ 10...
    .msg $nick 4[10MSG4]10:4 %xmsg. [ $+ [ $nick ] ]
    unset %acceptpm. [ $+ [ $nick ] ]
    notice %senick. [ $+ [ $nick ] ] 4 %xnick. [ $+ [ $nick ] ] 10has accepted your message.
  }
  else .msg $nick you dont have a message waiting for you
}
on *:text:!no:?:{
  if (%acceptpm. [ $+ [ $nick ] ] == on) {
    .msg $nick 10The message4 %senick. [ $+ [ $nick ] ] 10has been succesfully declined.
    .notice %senick. [ $+ [ $nick ] ] 4 %xnick. [ $+ [ $nick ] ] 10has declined your message.
  }
  else .msg $nick you dont have a message waiting for you
}