Use mail domain whitelist when sending notificatons
Use mail domain whitelist when sending notificatons

Summary:
Fixes T66279
Filtering is done just before send rather than while generating to make it easier to
verify that it always happens.

Test Plan: Existing unit, but can certainly add more.

Reviewers: paulruan

Reviewed By: paulruan

Subscribers: changesbot

Maniphest Tasks: T66279

Differential Revision: https://tails.corp.dropbox.com/D154035

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