HTML PHP jQuery Form

<form action="mail.php" id="contactform" method="post" name="contactform">
<input class="" name="name" placeholder="Name" required="" type="text"> <input class="" name="email" placeholder="Email" required="" type="email">
<textarea class="form-textarea" name="message" placeholder="Message" required=""></textarea>
<div class="checkbox">
<label><input id="checkBox" type="checkbox"> I'm a human</label>
<div id="validation_div" style="display:none">
<p class="captcha-text">Please retype the numbers into the box</p><img class="my_captcha" id="captcha_img" src="/captcha.php"> <input class="my_captcha_input" name="vercode" type="text">
<div class="refresh-captcha" id="refresh-captcha">try a different image</div>
</div>
</div>
<button class="btn-fill sign-up-btn" name="submit" type="submit">Send Message</button>
</form>
<div class="alert-success hidden" style="">
<span class="alert-text">Message Sent!</span> <span class="closebtn" onclick="this.parentElement.style.display='none';">x</span>
</div>
<div class="alert-error hidden" style="">
<span class="alert-text">Something went wrong, check the captcha or try again</span> <span class="closebtn" onclick="this.parentElement.style.display='none';">x</span>
</div>