Here is the technique that must be followed to retrieve deleted SMS. Required utility:

1) Any system explorer or file explorer program (eg. fileman or fexplorer.provided with the package)
Step 1) Open Fexplorer
Step 2) Select drive C: or D: depending on
the memory in which the Sms messages have been saved.
Step 3) For example u selected c: , Then open "system" folder.
step 4) Then in "system" folder open "mail" folder.
step 5) Then in this folder u will see different folders (eg 0010001_s) and certain files (eg 00100000). These files are the actual messages. Browse through every folder and open all files till u get the sms u are looking for.
Step 6) For opening the files don't choose options>file>open as it will show format not supported rather choose options>file>hex/text viewer. By selecting the "Hex/ text viewer" u ll be able to open and read the deleted SMS message