Delete deletes a trashed thread as well as its emails from DynamoDB and S3.

DetermineThread determines which thread an incoming email belongs to.

StoreEmail attempts to store the email.

StoreEmailWithExistingThread stores the email and updates the thread.

StoreEmailWithNewThread stores the email, creates a new thread, and add ThreadID to previous email.

Untrash marks an trashed email as not trashed.