easily transfer any of your messages already residing in the phone memory causing your phone to process various operations at a slow speed due to the memory usage by the sms messages.
Make sure that Messaging memory is set to Phone Memory (Messages -> Settings -> Other -> Memory -> Phone memory).
With FExplorer, go to E:\System\Mail and delete the whole dir. Not just the content, the whole dir.
Go to Messages -> Settings -> Other -> Memory and select “MMC”
Now you will be prompted if you would like to copy the messages to your “new” location.