How to send a bulk mail with attachment

//Sending Mail with attachement
MailMessage mail = new MailMessage();
mail.To ="me@mycompany.com;him@hiscompany.com;her@hercompany.com";
mail.From = "you@yourcompany.com";
mail.Subject = "this is a test email.";
mail.Body = "this is my test email body.";
MailAttachment attachment = new MailAttachment( Server.MapPath( "test.txt" ) ); 
mail.Attachments.Add( attachment );	//add the attachment
SmtpMail.SmtpServer = "smtp.gmail.com";  //your real server goes here
SmtpMail.Send( mail );