The one time use address are a unique token which maps to stored stated
in redis. We store the user_id, recipient_id, and subject. When an email
is received at this address it is sent to the stored recipient by the
stored user. Anyone with this address can send a single message as this
user.
(imported from commit 4219417bdc30c033a6cf7a0c7c0939f7d0308144)