Post submission of any message, SMSC at other end issues an unique messageid against every such message. In case message is not accepted by SMSC, it responds with error code in Command Id parameter. It may be noted that DLRs are retruned by SMSC against messageid.
Usually DLRs are issued as and when message is delivered on Mobile handset. Some times, it may be delayed.
Till the time DLR is received and updated, message status remains in Submitted mode. Eventually Submitted status means message is submitted to SMSC successfully and awaiting DLR from SMSC.
In case DLR is not received for any of the messages or taking longer time, same should be reported to SMSC.