Handle being BCCed
Instead of deducting the recipients based on the To / CC we could try to obtain it from the Received
header. To avoid deduplication we would need to store which recipient has received which message-id so far.
For parsing the received header https://github.com/alexwennerberg/crabmail/commit/1e6f63062f996551b379807d287059aeacf6c372 could be worth a look.