cb.js

    <lightning:input
            aura:id="emailField"
            type="email"
            label="{!$Label.c.BPEmail}"
            value="{!v.payment.email}"
            required="true"
            onchange="{!c.lowerCaseEmail}"
    />


 lowerCaseEmail: function (component, event, helper) {
    const emailInput = component.find('emailField');
    if ($A.util.isEmpty(emailInput)) return;

    const emailValue = component.get('v.payment.email');
    component.set('v.payment.email', emailValue.toLowerCase());

    if (/^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$/.test(emailValue)) {
        emailInput.setCustomValidity('');
    } else {
        emailInput.setCustomValidity('Invalid e-mail');
    }
},


validateEmail: function (email) {
    if (!email || email === '') return false;
    // var re = new RegExp('.+@.+\..+');
    var re = new RegExp(/^(([0-9a-z]+((\.|\-|\_)[0-9a-z]+)*))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/);
    return re.test(email);
},