One of the enhancements that Sony Mobile has packed inside the latest Xperia update for its devices was a new Messaging application, now available for non-updated handsets as well.
Although the aforementioned update was based on Android Jelly Bean, enthusiasts managed to extract the app from the update, and made it available for download for Ice Cream Sandwich Xperia smartphones.
The apk can be found over at the XDA-Developers, reportedly working on both rooted and non-rooted handsets.
Apparently, most of those who tried to install the app on their Xperia devices managed to successfully do so, which suggests that it should be safe for you to give it a try as well.
Those who would like to learn some more info on the port should head over to this forum thread over at the XDA-Developers.