如何使用c# windows应用程序发送大文件

我正在开发一个windows应用程序，我需要通过电子邮件发送一些文件作为附件。

public string SendMail(string mFrom,
        string mPass,
        string mTo,
        string mSub,
        string mMsg,
        string mFile,
        bool isDel)
    {
        string sql = "";
        try
        {
            System.Net.Mail.MailAddress mailfrom = new System.Net.Mail.MailAddress(mFrom);
            System.Net.Mail.MailAddress mailto = new System.Net.Mail.MailAddress(mTo);
            System.Net.Mail.MailMessage newmsg = new System.Net.Mail.MailMessage(mailfrom, mailto);
            newmsg.IsBodyHtml = false;
            if (mFile.Length > 2
                && File.Exists(mFile))
            {
                System.Net.Mail.Attachment att = new System.Net.Mail.Attachment(mFile);
                newmsg.Attachments.Add(att);
            }
            newmsg.Subject = mSub;
            newmsg.Body = mMsg;
            System.Net.Mail.SmtpClient smtp = new System.Net.Mail.SmtpClient("smtp.gmail.com", 587);
            smtp.UseDefaultCredentials = false;
            smtp.Credentials = new System.Net.NetworkCredential(mFrom, mPass);
            smtp.EnableSsl = true;
            smtp.Send(newmsg);
            newmsg.Dispose();
            GC.Collect();
            sql = "OK";
            if (isDel
                && File.Exists(mFile))
            {
                File.Delete(mFile);
            }
        }
        catch (Exception ex)
        {
            sql = ex.Message;
        }
        return sql;
    }

此代码适用于小文件。但是我需要发送1-2 GB的大文件。