MailboxOperator is a golang package for reading the emails in mbox and maildir format mailboxes and passing each email to a func meeting the interface `Operator` with the signature `(r io.Reader) error`. The package reads the provided mailboxes concurrently, and provides WorkersNum worker goroutines to run the `Operator` function. Shared resources used by the Operator should be safe for concurrent use. An error from any one call to the Operator will shut down both the workers and mailbox producer goroutines and return the first error. Reading xz, gz and bz2 compressed mbox files is supported transparently.