SqlCommand com1 = new SqlCommand(SELECT Email FROM EmailData, connection);
string email = com1.ExecuteScalar().ToString();
mail.Bcc.Add(email);
This code always return the first rows.You have to put it in loop except the execute query.
using (SqlDataReader dr = cmd.ExecuteReader())
{
while (dr.Read())
{
msg.CC.Add((string)dr["user_email"]);
}
}