magento-test/app/code/Magento/Checkout/Test/Mftf/ActionGroup/AssertStorefrontEmailValidationMessageOnCheckoutActionGroup.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
 /**
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
-->

<actionGroups xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
              xsi:noNamespaceSchemaLocation="urn:magento:mftf:Test/etc/actionGroupSchema.xsd">
    <actionGroup name="AssertStorefrontEmailValidationMessageOnCheckoutActionGroup">
        <annotations>
            <description>Validates that the provided Error Message appears on the Storefront Checkout page after an invalid Email Addressed is inputted.</description>
        </annotations>
        <arguments>
            <argument name="message" type="string" defaultValue="Please enter a valid email address (Ex: johndoe@domain.com)."/>
        </arguments>

        <waitForElementVisible selector="{{StorefrontCheckoutCheckoutCustomerLoginSection.emailErrorMessage}}" stepKey="waitForFormValidation"/>
        <see selector="{{StorefrontCheckoutCheckoutCustomerLoginSection.emailErrorMessage}}" userInput="{{message}}" stepKey="seeTheErrorMessageIsDisplayed"/>
    </actionGroup>
</actionGroups>