MemorySmtpServer.cs

public class MemorySmtpServer
{
    private readonly bool _redirectSmtpConfiguration;
    private const int Port = 62352;
    private readonly Configuration _configuration = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
    private MailSettingsSectionGroup _mailSettings;
    private string _originalHost;
    private int _originalPort;
    private SmtpServer _server;

    public MemorySmtpServer(bool redirectSmtpConfiguration)
    {
        _redirectSmtpConfiguration = redirectSmtpConfiguration;
    }

    public MemoryMessageSpool Messages { get; private set; }

    public void Start()
    {
        if (_redirectSmtpConfiguration)
        {
            _mailSettings = _configuration.GetSectionGroup("system.net/mailSettings") as MailSettingsSectionGroup;
            _originalHost = _mailSettings.Smtp.Network.Host;
            _originalPort = _mailSettings.Smtp.Network.Port;
            _mailSettings.Smtp.Network.Host = "127.0.0.1";
            _mailSettings.Smtp.Network.Port = Port;
            _configuration.Save();
        }
        Messages = new MemoryMessageSpool();
        _server = new SmtpServer(port: Port, messageSpool: Messages, recipientFilter: (context, address) => true);
        _server.Start();
    }

    public void Stop()
    {
        _server.Stop();
        if (_redirectSmtpConfiguration)
        {
            _mailSettings.Smtp.Network.Host = _originalHost;
            _mailSettings.Smtp.Network.Port = _originalPort;
            _configuration.Save();
        }
    } 
}