So the problem is that a motivated adversary can subpoena or simply
ask DoubleClick to hand over their IP/cookie logs. If you are using
Tor for /everything/, then what they get from DoubleClick for that
email address is just a Tor IP, no harm no foul. However, if the user
had set up a filter that only sends *yahoo.com through Tor, then
DoubleClick will have their /real IP/ on file in association with
whatever unique ID yahoo passed for that email address, even though
yahoo's records show only the Tor IP.