You can Add multi recipients to property MailMessage
C#:
var mail = new MailMessage();
mail.To.Add(new MailAddress("to@mail.com"));
For example: When you have a list with recipients, you can add them all
C#:
var mail = new MailMessage();
foreach (var item in list)
{
mail.To.Add(new MailAddress(item));
}
mail.From = new MailAddress("from@mail.com");
mail.Subject = "Subject";
mail.Body = "Body";
client.Send(mail);