Create your mail message before while loop.
Within the loop add to To or CC or BCC collections like this:
MailMessage message = new MailMessage();
string MessageFrom = "SSCReportAndAnalysis@telkom.co.za";
message.From = new MailAddress(MessageFrom);
string MessageTo = string.empty;
while (rdr.Read())
{
if (!rdr.IsDBNull(1))
{
updateID = rdr.GetString(1);
message.To.Add(new MailAddresss(updateID + "@telkom.co.za"));
}