Will a deactivated iPhone still receive iMessages and emails?
If your phone gets close to any WiFi network it has been connected to before it will reconnect and receive whatever is pending (iMessages, Mails etc.). Depending on your Notification Center settings and/or the strength of your password (if any) the person holding the phone will then also be able to read them.

But


are you sure the phone is gone and not just laying somewhere you haven't been looking at?
you still can use Find My iPhone to erase everything on the phone (which will be executed if the phone goes online again)