client_evDeliverSm event is invoking generally and gives the status for messages whether delivered or undeliverable after the message state enroute but in rare case it is not invoking and message state is only enroute and delivered status is not returned at all. I could not find out the reason ...

use sm.RegisteredDelivery = 1; after submition

foreach (SubmitSm sm in req)
{
sm.ValidityPeriod = DateTime.Now.AddDays(2).ToString("yyMMddhhmmss000+");
sm.RegisteredDelivery = 1;
}