Updated March 14, 2017
Information of contact groups created in the application and contacts added to those groups, either created manually or imported from device phone book, are stored in the device mass storage or SD card. The application does not transmit this data outside of the device.
The application does not store contents or recipients of messages after sending is finished, but on some Android versions sent and/or failed messages may appear in the device's main messaging application (Messages).
Anonymous usage data is collected for analytics purposes.