Let’s say I have this requirement to send the shipment email every 5 minutes or 10 minutes. To perform the task, I chose to create a custom cronjob and its file to achieve the functionality. Follow the below steps for more details.

Step 1: Create a crontab.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Cron:etc/crontab.xsd">
    <group id="default">
        <job name="sendshipmentemail" instance="MagemonkeysShipmentemailsendCronSend" method="execute">
            <schedule>0,15,30,45 * * * *</schedule>
        </job>
    </group>
</config>

Step 2: Create a Send.php file. Please follow the below code

<?php

namespace MagemonkeysShipmentemailsendCron;

use MagentoFrameworkApiSearchFilterGroupBuilder;
use MagentoFrameworkApiSearchCriteriaBuilder;
use MagentoFrameworkAppObjectManager;
use MagentoSalesModelOrderShipmentRepository;
use MagentoSalesModelOrderRepository;
use MagentoFrameworkApiFilterBuilder;
use PsrLogLoggerInterface;

class Send
{

    private $orderRepository;
    private $searchCriteriaBuilder;

    public function __construct(

        OrderRepository $orderRepository,
        SearchCriteriaBuilder $searchCriteriaBuilder,
        ShipmentRepository $shipmentRepository,
        FilterBuilder $filterBuilder,
        FilterGroupBuilder $filterGroupBuilder,
        LoggerInterface $logger
    )
    {
        $this->orderRepository = $orderRepository;
        $this->shipmentRepository = $shipmentRepository;
        $this->searchCriteriaBuilder = $searchCriteriaBuilder;
        $this->filterBuilder = $filterBuilder;
        $this->filterGroupBuilder = $filterGroupBuilder;
        $this->objectManager = ObjectManager::getInstance();
        $this->logger = $logger;
    }


    public function execute()
    {

        $filtergroup_date = $this->filterGroupBuilder
            ->addFilter(
                $this->filterBuilder
                    ->setField('created_at')
                    ->setConditionType('gt')
                    ->setValue('2020-01-01')
                    ->create()
            )
            ->create();


        $filtergroup_email_sent = $this->filterGroupBuilder
            ->addFilter(
                $this->filterBuilder
                    ->setField('email_sent')
                    ->setConditionType('null')
                    ->create()
            )
            ->create();

        $filtergroup_send_email = $this->filterGroupBuilder
            ->addFilter(
                $this->filterBuilder
                    ->setField('send_email')
                    ->setConditionType('null')
                    ->create()
            )
            ->create();

        $searchCriteria = $this->searchCriteriaBuilder
            ->setFilterGroups([
                $filtergroup_date,
                $filtergroup_email_sent,
                $filtergroup_send_email
            ])
            ->create();;

        $shipmentList = $this->shipmentRepository->getList($searchCriteria)->getItems();

        foreach ($shipmentList as $shipment) {
            if (count($shipment->getTracks()) > 0) {
                $this->objectManager->create('MagentoShippingModelShipmentNotifier')->notify($shipment);
                $shipment->setEmailSent("1");
                $shipment->save();
            } else {
                $this->logger->warning("Unable to send shipment email for order");
            }
        }
    }
}

Now, when the cron runs it will execute our code and if any shipment mail needs to send then it will be sent.