Issues with mass subscriptions from @yahoo.com addresses

Hi,
We have noticed a surge in activity from @yahoo.com addresses on our
mailing lists, with hundreds of users mass subscribing. It's unclear what
the goal could be, either setting the stage to send spam in the future, or
simply collecting email addresses from mailing list users.

To minimize the risk, we have:
* Removed all the users with @yahoo.com, trying to exclude known
contributors.
* Switched the subscription settings to "Confirm and approve", meaning that
a new subscriber will need to both confirm the subscription by clicking a
link sent to them, and get approval from a moderator. This will inevitably
delay the process of subscribing to a mailing list.
* Banned @yahoo.com addresses from subscribing.

The last step was necessary, since the volume (about 60 subscriptions in 12
hours for dev-l10n) was making it impossible to act on real users trying to
subscribe.

If you see this message on other channels and found yourself unsubscribed
by mistake, or are aware of contributors using @yahoo.com addresses willing
to subscribe, please get in touch directly, since we can manually subscribe
users, overriding the domain block.

Sorry for any possible inconvenience caused by this change.

Francesco