What are the delivery issues due to third party application?
Please note that in case of GSM, the technology itself does not allow the DLR getting generated from the operator. In case of GSM the DLR has to come from the handset and until then the operator retries. In case of CDMA the SMSC sends the DLR. Thus for GSM number there are no chances of fake DLR.
The observations you faced could be because of any or multiple of the following reasons to our experience:
1. The handset has the primary inbox as an application and not the phone inbox
2. The handset has blocked the sender-id
a) Either user had accidentally got the sender blocked
b) Either the user’s application got the SMS or sender blocked.