I've just got this problem recently.
I try to text to my friend via MyPhoneExplorer, the message is still gone delivered. However, there is no record of that message in the phone or in the inbox section in the program.
I tried to uninstall and install it again and the problem is fixed, but just for a while. After a day, it repeats again
Yeah. I'm pretty sure about this.
But I've just uninstalled the program and reinstalled it recently. The problem has not appeared until now.
I will tell you asap if it repeats
I have had a similar problem on 3 occassions. Sometimes a message hangs and will not send (although i think it does actually send) and you have to crash out of MPE. Next time you send a mesaage the box is un-ticked. I haven't really studied the issue but i'm sure that's what happens.