I have a sandboxed app.
This app has an XPC service which is not sandboxed.
Does the XPC service need to be sandboxed as well for me to be able to submit to the Mac App Store?
Every binary, including XPC components, that's part of a Mac App Store submission must be sandboxed. However the entitlements for each binary can (and probably should, if the app is well-factored) be different: XPC is designed to support that case.