getDeliveryDeadline(email) {
        // Return undefined if targetDeliveryWindow is 0 (or less)
        const targetDeliveryWindow = this.#sendingService.getTargetDeliveryWindow();
        if (targetDeliveryWindow === undefined || targetDeliveryWindow <= 0) {
            return undefined;
        }
        try {
            const startTime = email.get('created_at');
            const deadline = new Date(startTime.getTime() + targetDeliveryWindow);
            return deadline;
        } catch (err) {
            return undefined;
        }
    }
Returns the sending deadline for an email
Based on the email.created_at timestamp and the configured target delivery window
@param {*} email 
@returns Date | undefined