You are sending an SMS to someone, but are you sure that the recipient receives it?
There is an Option on major s60 device called DELIVERY REPORTS. It shows status if your messages are received or not.
By default, it is off. You can turn this on by going to Messaging | Message Settings | Text Messages| Receive Reports. You can see the reports on Reports folder.
It is useful when your network providers delays up your sms sending/receiving (just what I am suffering). It might not help it but you`ll get to know the reports.