If, you want to send another contact mail copy to another email address as a BCC? Then follow the below steps.

Step 1: Create a registration file like Magemonkeys/ContactCc/registration.php

<?php
MagentoFrameworkComponentComponentRegistrar::register(
    MagentoFrameworkComponentComponentRegistrar::MODULE,
    'Magemonkey_ContactCc',
    __DIR__
);

Step 2: Create di file like Magemonkeys/ContactCc/etc/di.xml

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="MagentoContactControllerIndexPost" type="MagemonkeyContactCcRewriteMagentoContactControllerIndexPost"/>
</config>

Step 3: Create module file like Magemonkeys/ContactCc/etc/module.xml

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Magemonkey_ContactCc" setup_version="1.0.2"/>
</config>

Step 4: Create a system file like Magemonkeys/ContactCc/etc/adminhtml/system.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <section id="contact" translate="label" type="text" sortOrder="100" showInDefault="1" showInWebsite="1" showInStore="1">
            <tab>general</tab>
            <group id="email" translate="label" type="text" sortOrder="50" showInDefault="1" showInWebsite="1" showInStore="1">
                <label>Email Options</label>
                <field id="copy_to" translate="label comment" type="text" sortOrder="200" showInDefault="1" showInWebsite="1" showInStore="1">
                    <label>Send Contact Us Email Copy To</label>
                    <comment>Comma-separated</comment>
                </field>                
            </group>
        </section>
    </system>
</config>

Step 5: Create helper file like Magemonkeys/ContactCc/Helper/Email.php

<?php

namespace MagemonkeyContactCcHelper;

use MagentoFrameworkAppHelperAbstractHelper;
use MagentoFrameworkAppHelperContext;
use MagentoStoreModelStoreManagerInterface;
use MagentoFrameworkAppConfigScopeConfigInterface;

/**
 * Class Email
 * @package MagemonkeyContactCcHelper
 */
class Email extends AbstractHelper
{
    const XML_PATH_EMAIL_COPY_METHOD = 'contact/email/copy_method';
    const XML_PATH_EMAIL_COPY_TO = 'contact/email/copy_to';

    /**
     * @var StoreManagerInterface
     */
    private $storeManager;

    /**
     * @var ScopeConfigInterface
     */
    protected $scopeConfig;

    /**
     * Email constructor.
     * @param Context $context
     * @param StoreManagerInterface $storeManager
     * @param ScopeConfigInterface $scopeConfig
     */
    public function __construct(
        Context $context,
        StoreManagerInterface $storeManager,
        ScopeConfigInterface $scopeConfig
    ) {
        $this->storeManager = $storeManager;
        $this->scopeConfig = $scopeConfig;
        parent::__construct($context);
    }

    /**
     * @return array|bool
     * @throws MagentoFrameworkExceptionNoSuchEntityException
     */
    public function getEmailCopyTo()
    {
        $data = $this->getConfigValue(self::XML_PATH_EMAIL_COPY_TO, $this->storeManager->getStore()->getId());
        if (!empty($data)) {
            return explode(',', $data);
        }
        return false;
    }

    /**
     * @return mixed
     * @throws MagentoFrameworkExceptionNoSuchEntityException
     */
    public function getCopyMethod()
    {
        return $this->getConfigValue(self::XML_PATH_EMAIL_COPY_METHOD, $this->storeManager->getStore()->getId());
    }

    /**
     * @param $xmlPath
     * @param null $storeId
     * @return mixed
     */
    public function getConfigValue($xmlPath, $storeId = null)
    {
        return $this->scopeConfig->getValue(
            $xmlPath,
            MagentoStoreModelScopeInterface::SCOPE_STORE,
            $storeId
        );
    }
}

Step 6: Create PHP file like Magemonkey/ContactCc/Rewrite/Magento/Contact/Controller/Index/Post.php

<?php

namespace MagemonkeyContactCcRewriteMagentoContactControllerIndex;

use MagentoFrameworkAppArea;
use MagentoContactModelConfigInterface;
use MagentoFrameworkAppActionContext;
use MagentoFrameworkDataObject;
use MagentoFrameworkAppRequestDataPersistorInterface;
use MagemonkeyContactCcHelperEmail;
use PsrLogLoggerInterface;
use MagentoFrameworkExceptionLocalizedException;
use MagentoContactModelMailInterface;
use MagentoFrameworkAppObjectManager;
use MagentoFrameworkTranslateInlineStateInterface;
use MagentoStoreModelStoreManagerInterface;
use MagentoFrameworkMailTemplateTransportBuilder;

/**
 * Class Post
 * @package MagemonkeyContactCcRewriteMagentoContactControllerIndex
 */
class Post extends MagentoContactControllerIndexPost
{
    /**
     * @var Context
     */
    private $context;

    /**
     * @var ConfigInterface
     */
    private $contactsConfig;

    /**
     * @var MailInterface
     */
    private $mail;

    /**
     * @var DataPersistorInterface
     */
    private $dataPersistor;

    /**
     * @var LoggerInterface
     */
    private $logger;

    /**
     * @var Email
     */
    private $helper;

    /**
     * @var StateInterface
     */
    private $inlineTranslation;

    /**
     * @var TransportBuilder
     */
    private $transportBuilder;

    /**
     * @var StoreManagerInterface
     */
    private $storeManager;

    /**
     * Post constructor.
     * @param Context $context
     * @param ConfigInterface $contactsConfig
     * @param MailInterface $mail
     * @param DataPersistorInterface $dataPersistor
     * @param Email $helper
     * @param LoggerInterface|null $logger
     * @param TransportBuilder $transportBuilder
     * @param StateInterface $inlineTranslation
     * @param StoreManagerInterface|null $storeManager
     */
    public function __construct(
        Context $context,
        ConfigInterface $contactsConfig,
        MailInterface $mail,
        DataPersistorInterface $dataPersistor,
        Email $helper,
        LoggerInterface $logger = null,
        TransportBuilder $transportBuilder,
        StateInterface $inlineTranslation,
        StoreManagerInterface $storeManager = null
    ) {
        $this->context = $context;
        $this->contactsConfig = $contactsConfig;
        $this->mail = $mail;
        $this->dataPersistor = $dataPersistor;
        $this->logger = $logger ?: ObjectManager::getInstance()->get(LoggerInterface::class);
        $this->helper = $helper;
        $this->transportBuilder = $transportBuilder;
        $this->inlineTranslation = $inlineTranslation;
        $this->storeManager = $storeManager ?: ObjectManager::getInstance()->get(StoreManagerInterface::class);
        parent::__construct($context, $contactsConfig, $mail, $dataPersistor, $logger);
    }

    /**
     * Contact store
     * @return MagentoFrameworkControllerResultRedirect
     */
    public function execute()
    {
        if (!$this->getRequest()->isPost()) {
            return $this->resultRedirectFactory->create()->setPath('*/*/');
        }
        try {
            $this->sendEmail($this->validatedParams());
            $this->messageManager->addSuccessMessage(
                __('Thanks for contacting us with your comments and questions. We'll respond to you very soon.')
            );
            $this->dataPersistor->clear('contact_us');
        } catch (LocalizedException $e) {
            $this->messageManager->addErrorMessage($e->getMessage());
            $this->dataPersistor->set('contact_us', $this->getRequest()->getParams());
        } catch (Exception $e) {
            $this->logger->critical($e);
            $this->messageManager->addErrorMessage(
                __('An error occurred while processing your form. Please try again later.')
            );
            $this->dataPersistor->set('contact_us', $this->getRequest()->getParams());
        }
        return $this->resultRedirectFactory->create()->setPath('contact/index');
    }

    /**
     * Send email
     * @param array $post
     * @throws MagentoFrameworkExceptionMailException
     * @throws MagentoFrameworkExceptionNoSuchEntityException
     */
    public function sendEmail($post)
    {
        $this->send(
            $post['email'],
            ['data' => new DataObject($post)]
        );
    }

    /**
     * Send email
     * @param $replyTo
     * @param array $variables
     * @throws MagentoFrameworkExceptionMailException
     * @throws MagentoFrameworkExceptionNoSuchEntityException
     */
    public function send($replyTo, array $variables)
    {
        $replyToName = !empty($variables['data']['name']) ? $variables['data']['name'] : null;

        $this->inlineTranslation->suspend();

        try {
            $this->configureEmailTemplate($replyTo, $replyToName, $variables);
            $this->transportBuilder->addTo($this->contactsConfig->emailRecipient());

            $copyTo = $this->helper->getEmailCopyTo();
            if (!empty($copyTo) && $this->helper->getCopyMethod() == 'bcc') {
                foreach ($copyTo as $email) {
                    $this->transportBuilder->addBcc($email);
                }
            }

            $transport = $this->transportBuilder->getTransport();
            $transport->sendMessage();

            $this->sendCopyTo($replyTo, $replyToName, $variables);
        } catch (Exception $e) {
            $this->logger->error($e->getMessage());
        } finally {
            $this->inlineTranslation->resume();
        }
    }

    /**
     * Prepare and send copy email message
     * @param string $replyTo
     * @param string $replyToName
     * @param array $variables
     * @return void
     */
    public function sendCopyTo($replyTo, $replyToName, $variables)
    {
        $copyTo = $this->helper->getEmailCopyTo();
        if (!empty($copyTo) && $this->helper->getCopyMethod() == 'copy') {
            foreach ($copyTo as $email) {
                $this->configureEmailTemplate($replyTo, $replyToName, $variables);
                $this->transportBuilder->addTo($email);
                $transport = $this->transportBuilder->getTransport();
                $transport->sendMessage();
            }
        }
    }

    /**
     * Configure email template
     * @param string $replyTo
     * @param string $replyToName
     * @param array $variables
     * @return void
     */
    protected function configureEmailTemplate($replyTo, $replyToName, $variables)
    {
        $this->transportBuilder->setTemplateIdentifier($this->contactsConfig->emailTemplate());
        $this->transportBuilder->setTemplateOptions([
            'area' => Area::AREA_FRONTEND,
            'store' => $this->storeManager->getStore()->getId()
        ]);
        $this->transportBuilder->setTemplateVars($variables);
        $this->transportBuilder->setFrom($this->contactsConfig->emailSender());
        $this->transportBuilder->setReplyTo($replyTo, $replyToName);
    }

    /**
     * Validate form params
     * @return array
     * @throws Exception
     */
    private function validatedParams()
    {
        $request = $this->getRequest();
        if (trim($request->getParam('name')) === '') {
            throw new LocalizedException(__('Name is missing'));
        }
        if (trim($request->getParam('comment')) === '') {
            throw new LocalizedException(__('Comment is missing'));
        }
        if (false === strpos($request->getParam('email'), '@')) {
            throw new LocalizedException(__('Invalid email address'));
        }
        if (trim($request->getParam('hideit')) !== '') {
            throw new LocalizedException(__('Error'));
        }
        return $request->getParams();
    }
}

Step 7: Then after run the below commands.

php bin/magento setup:upgrade
php bin/magento setup:static-content:deploy
php bin/magento cache:flush

That’s it…

Now clean cache and check in Admin > STORES > Settings > Configuration > GENERAL > Contacts. Your new email address field option appear here.