Yahoo operates a large family of consumer domains (AOL, AT&T, Verizon,
Frontier, sbcglobal, bellsouth, etc.) that aggressively defer cold senders
with 421 'unexpected volume / user complaints', which poisons our self-hosted
sending IP for every other provider. Previously we only excluded
aol.com/yahoo.com/ymail.com/rocketmail.com.
Centralize the authoritative block list in scripts/_email_exclusions.py and
import it from both audience builders so they stay in sync.
