How to send an email with expiry date in asp.net C# ?

I have a requirement to send bulk emails to users  for the marketing purpose where email should expire after 30 days, Please help me where i can add the expire date in the email contents, I am using C# for programming , My codes are as below

 

MailMessage mail = new MailMessage();                 
SmtpClient SmtpServer = new SmtpClient("smtp.gmail.com");                  
mail.From = new MailAddress("[email protected]");                 
mail.To.Add("to_address");                 
mail.Subject = "Test Mail";                 
mail.Body = "This is for testing SMTP mail from GMAIL";                  
SmtpServer.Port = 587;                 
SmtpServer.Credentials = new System.Net.NetworkCredential("username", "password");
SmtpServer.EnableSsl = true;                  
SmtpServer.Send(mail);                 
MessageBox.Show("mail Send");