A simple plugin that allows for players to send private messages to one another, and reply to the last person who they messaged or received a message from.
This plugin also has 'social spy' which allows for moderation of these messages.

• /msg (or dm, m, pm, tell, t)
  Allows the user to send a private message to another player.
  Usage: /msg <recipient> <message>.
  This command requires the spm.message.send permission.

• /reply (or /r, /re):
  Allows the user to reply to a previously received private message.
  Usage: /r <message>.
  This command also requires the spm.message.send permission.

• /socialspy (or /ss, /spy, /sspy):
  Toggles socialspy for the user.
  Allows staff members to read other people's private messages.
  This command requires the spm.socialspy.toggle permission.

• /spmreload:
  Reloads the SimplePM configuration.
  This command requires the spm.reload permission.

• spm.socialspy
  Allows a player to read other player's private messages.
  Default: op
• spm.socialspy.toggle:
  Allows a player to toggle socialspy on and off
  Default: op
• spm.recipient.override
  Allows a player to send players a direct message whether they have the receive permission or not. Useful for staff.
  Default: op
• spm.consolespy:
  Allows a player to read messages being sent from the console directly to players
  Default: op
• spm.reload:
  Allows a player to reload the plugin configuration/locale.
  Default: op
• spm.message.send:
  Allows a player to send direct messages with /msg and /r.
  Default: true
• spm.message.receive
  Allows a player to receive direct messages
  Default: true

You can use placeholderAPI placeholders in the messages by doing: <papi:placeholder>
You will need to remove any % from the placeholder, and put in the text alone, so for the placeholder %player_displayname% you would use <papi:player_displayname> in the file here.
You will need placeholderAPI installed for those placeholders to work