If it is mail enabled, one must do following which I did with my Moto Q 8 GSM
- Sync your contact books with Exchange server or mail server.
- Delete all mails one by one.
- Unconfigure any mails which can be unconfigured. Or delete the account.
- Take backup of the whole mobile on computer using activesync or any sync option.
- Take out the memory card.
- Take out the SIM.
- Do Master reset on device and reboot it. master reset brings mobile back to factory setting stage where no user data is present.
Now, it can be safely given to Service centre guys.