NVC will email or send mail to the petitioner after processing a

NVC will email or mail welcome letter to the petitioner after processing approved petition from USCIS first.

You can also email NVC asking your cases if your (petitioner) email/address is out of date and the PD is current. 

https://travel.state.gov/content/travel/en/us-visas/visa-information-resources/ask-nvc.html