Well, i found a solution myself.

string connectionString = WebConfigurationManager.ConnectionStrings["umbracoConnectionString"].ConnectionString;
SqlConnection conn = new SqlConnection(connectionString);SqlCommand cmdGroup = new SqlCommand("SELECT Email FROM nyhedsTilmelding WHERE Titel=@Titel", conn);
SqlCommand cmdAll = new SqlCommand("SELECT Email FROM nyhedsTilmelding", conn);

cmdGroup.Parameters.Add("@Titel", System.Data.SqlDbType.NVarChar, 50);
cmdGroup.Parameters["@Titel"].Value = CategoryDropDownList.SelectedValue;

if (CategoryDropDownList.SelectedValue == "Alle")
{
    cmdAll.Connection.Open();
        SqlDataReader rdr = cmdAll.ExecuteReader();
        if (rdr.HasRows)
        {
            List<string> emailAll = new List<string>();
                while (rdr.Read())
                {
                    emailAll.Add(rdr["Email"].ToString());
                        emailAll.Distinct().ToList();
                }
                string strEmailAll = string.Join(", ", emailAll.ToArray());
                SendButton.NavigateUrl = "mailto:?subject=Nyhedsmail fra Oldengard.dk&bcc=" + strEmailAll;
                rdr.Close();
                cmdAll.Connection.Close();
                cmdAll.Connection.Dispose();
    }
        else
        {
            SendButton.NavigateUrl = "mailto:"; //just empty link with mailto, could remove the link instead
        }
}
else
{
    cmdGroup.Connection.Open();
        SqlDataReader reader = cmdGroup.ExecuteReader();
        if (reader.HasRows)
        {
            List<string> emailGroup = new List<string>();
            while (reader.Read())
                {
                    emailGroup.Add(reader["Email"].ToString());
                        emailGroup.Distinct().ToList();
                }
                string strEmailGroup = string.Join(", ", emailGroup.ToArray());
                SendButton.NavigateUrl = "mailto:?subject=Nyhedsmail fra Oldengard.dk&bcc=" + strEmailGroup;
                reader.Close();
                cmdGroup.Connection.Close();
            cmdGroup.Connection.Dispose();
        }
    else
        {
            SendButton.NavigateUrl = "mailto:"; //just empty link with mailto, could remove the link instead

        }
}

- Bjarne