• HTML
• CSS
• JS

HTML

<!--FontAwesome Icons-->
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.15.2/css/all.min.css">
<!--Google Fonts-->
<link href="https://fonts.googleapis.com/css2?family=Poppins:wght@400;500&display=swap" rel="stylesheet">

<div class="container">
    <label for="email-id">Email Id</label><br>
    <input type="email" id="email-id" oninput="checker()">
    <div id="icon"></div>
    <p id="error-msg">Please Enter A Valid Email Id</p>
</div>

CSS

*,
*:before,
*:after{
    padding:0;
    margin:0;
    box-sizing:border-box;
}
body{
    height:100vh;
    background:linear-gradient(
        135deg,
        #61d954,
        #2ebf75
    );
}
.container{
    width:400px;
    background-color:#ffffff;
    padding:50px 30px;
    position:absolute;
    transform:translate(-50%,-50%);
    top:50%;
    left:50%;
    border-radius:10px;
    box-shadow:25px 25px 30px rgba(0,0,0,0.15);
    color:#111111;
}
label,
input,
#error-msg{
    font-family:'Poppins',sans-serif;
}
label{
    display:inline-block;
    font-weight:500;
    margin-bottom:10px;
}
input[type="email"]{
    display:inline-block;
    border:2px solid #d1d3d4;
    width:88%;
    height:50px;
    border-radius:5px;
    outline:none;
    letter-spacing:0.5px;
    font-weight:400;
}
#icon{
    float:right;
    height:50px;
    position:relative;
    font-size:25px;
    padding-top:10px;
}
#error-msg{
    display:none;
    color:#ff2851;
    font-size:14px;
    margin-top:10px;
}

JS

let emailId = document.getElementById("email-id");
let errorMsg = document.getElementById("error-msg");
let icon = document.getElementById("icon");
let mailRegex = /^[a-zA-Z][a-zA-Z0-9\-\_\.]+@[a-zA-Z0-9]{2,}\.[a-zA-Z0-9]{2,}$/;

function checker(){
    icon.style.display = "inline-block";
    if(emailId.value.match(mailRegex)){
        icon.innerHTML = '<i class="fas fa-check-circle"></i>';
        icon.style.color = '#2ecc71';
        errorMsg.style.display = 'none';
        emailId.style.border = '2px solid #2ecc71';
    }
    else if(emailId.value == ""){
        icon.style.display = 'none';
        errorMsg.style.display = 'none';
        emailId.style.border = '2px solid #d1d3d4';
    }
    else{
        icon.innerHTML = '<i class="fas fa-exclamation-circle"></i>'
        icon.style.color = '#ff2851';
        errorMsg.style.display = 'block';
        emailId.style.border = '2px solid #ff2851';
    }
}

/* All the credits: https://www.youtube.com/watch?v=1tnINeBLNj4*/