public interface IMailSendLogic
{
    void SendMailAsync(string to, string from, string subject, string text);
}

public class MailSendLogic : IMailSendLogic
{
    private string SmtpHost = "";
    private int SmtpPort = 587;
    private string User = "";
    private string Pass = "";
    public async void SendMailAsync(string to, string from, string subject, string text)
    {
        var message = new MimeKit.MimeMessage();
        // 送り元情報  
        message.From.Add(new MimeKit.MailboxAddress("<送信者名>", from));
        // 宛先情報  
        message.To.Add(new MimeKit.MailboxAddress("<宛名>", to));
        // 表題  
        message.Subject = subject;
        // 内容  
        var textPart = new MimeKit.TextPart(MimeKit.Text.TextFormat.Plain);
        textPart.Text = text;
        message.Body = textPart;

        using (var client = new MailKit.Net.Smtp.SmtpClient())
        {
            try
            {
                // SMTPサーバに接続  
                await client.ConnectAsync(SmtpHost, SmtpPort);
                Debug.WriteLine("接続完了");

                // SMTPサーバ認証（あれば）  
                await client.AuthenticateAsync(User, Pass);

                // 送信  
                await client.SendAsync(message);

                // 切断  
                await client.DisconnectAsync(true);

            }
            catch (Exception e)
            {
                Debug.WriteLine("{0} Exception caught.", e);
            }
        }
    }
}