def delete_oldest(self):
        """ Delete all the oldest duplicates.

        Keeps all mail of the duplicate set but those sharing the oldest
        timestamp.
        """
        logger.info(
            "Deleting all mails sharing the oldest {} timestamp...".format(
                self.oldest_timestamp))
        # Select candidates for deletion.
        candidates = [
            mail for mail in self.pool
            if mail.timestamp == self.oldest_timestamp]
        if len(candidates) == self.size:
            logger.warning(
                "Skip deletion: all {} mails share the same timestamp."
                "".format(self.size))
            return
        logger.info(
            "{} candidates found for deletion.".format(len(candidates)))
        for mail in candidates:
            self.delete(mail)
Delete all the oldest duplicates.

        Keeps all mail of the duplicate set but those sharing the oldest
        timestamp.