public function addRecipientVariable(
        string $recipient,
        string $recipientVariable,
        string $recipientVariableValue
    ): self {
        if (! isset($this->recipientVariables[$recipient])) {
            $this->recipientVariables[$recipient] = [];
        }

        $this->recipientVariables[$recipient][$recipientVariable] = $recipientVariableValue;

        return $this;
    }