I have been frustrated with the performance of the Exchange TouchDown for Andriod after Symantec bought it out in 2016. When the Exchange TouchDown app was developed and maintained by NitroDesk it was really top notch. Symantec really made a mess of the app. Instead of fixing it, they are shutting it down.
Symantec released an end of life statement "Symantec Corporation hereby informs you that we will be discontinuing the availability of Touchdown by Symantec. End of Product Life (EOL) 7/3/2017"
THERE IS GOOD NEWS: I have been using the last version 8.5.00086 before Symantec made a huge mess of the Exchange TouchDown for Andriod app. An alternative to NitroDesk you can use the Outlook app by Mircorsoft for Android (personally I still prefer the NitroDesk TouchDown for Andriod app).
Touchdown Before Symantec: You can download the APK of the last version published by NitroDesk from the link below. This is the free version. If you paid for the full version your key will still work since this is the last version developed by NitroDesk with NO Symantec features at all!
com.nitrodesk.touchdown_v8.5.apk (9.13 mb)