It seems you are adding multiple addresses to the same MailMessage
on each iteration. You need to clear the message.To
collection or create a new message in each iteration
while (read.Read())
{
string mail = read["Mail"].ToString();
try
{
message.To.Clear();
message.To.Add(mail);
//email
smtp.Send(message);
}
catch
{
MessageBox.Show("Text");
}
}