Yup, same. I'm blocking bc.googleusercontent.com and also firebaseapp.com for now. The reverse DNS should also be able to be used, as the fakey spam domains don't match up with the PTR record, but I want to wait until I can watch the logs for a bit to make sure that works nicely.

e.g.

PTR: 53.220.83.34.bc.googleusercontent.com

HELO: sax.co.uk