Now if you wish to flash the .23 firmware straight away using Flashtool, you can't, it'll will fail, I found this issue yesterday, I dunno if it has been mentioned anywhere.
To counter it, is easy:
1.Use a compression program on your PC to open the FTF(I would recommend 7-Zip)
2.Extract kernel.sin from it.
3.Now connect you Xperia E in fastboot mode(Vol up+USB cable connected)
4.Hit the "Select kernel to flash" and flash the kernel we extracted in step 2.
5.After the successful flash you can now revert to stock[.23 firmware] with no problems.
Hit thanks if I helped