source: binary-improvements/7dtd-server-fixes/src/TelnetCommands/Reply.cs@ 112

using System;
using System.Collections.Generic;

public class Reply : ConsoleCommand
{
        public Reply (ConsoleSdtd cons) : base(cons)
        {
        }

        public override string Description ()
        {
                return "send a message to  the player who last sent you a PM";
        }

        public override string[] Names ()
        {
                return new string[] { "reply", "re" };
        }

        private void SendMessage (ClientInfo _receiver, ClientInfo _sender, string _message)
        {
                PrivateMassageConnections.SetLastPMSender (_sender, _receiver);
                string senderName = CommonMappingFunctions.GetPlayerName (_sender);

                CommonMappingFunctions.GetConnectionManager ().networkView.RPC ("RPC_ChatMessage", _receiver.networkPlayer,
                                new object[] {
                        _message,
                        -1,
                        senderName + " (PM)",
                        true
                }
                );
                string receiverName = CommonMappingFunctions.GetPlayerName (_receiver);
                m_Console.SendResult ("Message to player " + (receiverName != null ? "\"" + receiverName + "\"" : "unknownName") + " sent with sender \"" + senderName + "\"");
        }

        private void RunInternal (ClientInfo _sender, string[] _params)
        {
                if (_params.Length < 1) {
                        m_Console.SendResult ("Usage: reply <message>");
                        return;
                }

                string message = _params [0];
                for (int i = 1; i < _params.Length; i++) {
                        message += " " + _params [i];
                }

                ClientInfo receiver = PrivateMassageConnections.GetLastPMSenderForPlayer (_sender);
                if (receiver != null && CommonMappingFunctions.GetClientID (receiver) >= 0) {
                        SendMessage (receiver, _sender, message);
                } else {
                        if (receiver != null) {
                                m_Console.SendResult ("The sender of the PM you last received is currently not online.");
                        } else {
                                m_Console.SendResult ("You have not received a PM so far.");
                        }
                }
        }

        public override void ExecuteRemote (string _sender, string[] _params)
        {
                try {
                        m_Console.SendResult (string.Format ("{0} executing remote command '{1}' {2}", _sender, this.Names () [0], string.Join (" ", _params)));
                        ClientInfo ci = CommonMappingFunctions.GetClientInfoFromSteamID (_sender);
                        RunInternal (ci, _params);
                } catch (Exception e) {
                        Log.Out ("Error in Reply.ExecuteRemote: " + e);
                }
        }

        public override void Run (string[] _params)
        {
                Log.Out ("Command \"reply\" can only be used on clients!");
        }
}