gistfile1.cs

public interface IMailService
{
    void Send(string sender, string recipient, string subject, string body);
}
 
public class MessageEntity
{
    //...
    public void SendThrough(IMailService mailService)
    {
        string subject = isReply ? "Re : " + title : title;
        mailService.Send(sender, recipient, subject, GetMessageBody(content));
    }
}