how to use other SMTP send email in C#

private string SendMail(string from, string fromname, string to, string subject, string body, string username, string password, string server)
    {
        
        try
        {
            MailMessage mail = new MailMessage();
            mail.From = new MailAddress(from, fromname);
            mail.To.Add(to);
            mail.Subject = subject;
            mail.Body = body;
            
            SmtpClient smtp = new SmtpClient();
            smtp.Host = "smtp.gmail.com";
            smtp.Port = 587;

            smtp.Credentials = new System.Net.NetworkCredential(username, password);
            smtp.EnableSsl = true;
            smtp.Send(mail);

            return "send ok";
        }
        catch (Exception exp)
        {
            return exp.Message;
        }
    }

    protected void btnConfirm_Click(object sender, EventArgs e)
    {
        SendMail("xxx@gmail.com", "Lucy", "xxxx@gmail.com", "text", "this is a test email", "xxx@gmail.com", "xxx", "smtp.gmail.com");  
    }