public function getEnvelope(): Envelope
    {
        if (null === $this->envelope) {
            throw new MissingEnvelopeException('No Envelope provided! Provide one beforehand');
        }

        // Check for recipients
        if (empty($this->envelope->getRecipients())) {
            throw new MissingRecipientException('No recipients provided! Add them beforehand');
        }

        return $this->envelope;
    }