NewPickupFolderQueue returns an implementation of MailQueueDispatcher that watches a mailqueue folder and consumes messages that are left there and if it fails to consume the message, moves the message to a badmail folder.

MailQueueDispatcher describes the interface a dispatcher must fulfill in order to use the MailQueueCallbackFn.

MailQueueCallbackFn describes the callback mechanism the dispatcher uses to transmit raw bytes representing an email to the mailer to actually send.