using System;

using System.Net.Mail;

public class Program

{

    public static void Main()

    {

        var message = new MailMessage

        {

            To = {new MailAddress("me@test.nl")},

                Subject = "Test Yay",

            Body = "Hallo",

            Sender = new MailAddress("sender@test.nl"),

            From = new MailAddress("from@test.nl"),

            ReplyToList = {new MailAddress("replyTo@test.nl")},

        };

        message.Headers["Return-Path"] = "returnPath@test.nl";

        Console.WriteLine($"sender: {message.Sender}, returnpath: {message.Headers["Return-Path"]}");

    }

}