How does this Form check for a valid email address

[ChrisjChrisj]

This Form works successfully and won’t submit without a proper email address format in the email field. A message pops-up: “Fill out this field” when a Submit attempt is made when the field is blank.
And when garbage text is entered a pop-up message shows “Enter an email address”.
I haven’t looked at this Form in quite a while, maybe you can help me to determine where to find out how this Form checks for a proper email address:

<form action='submit1.php' method='post' name='myform' onSubmit="return checkemail()">
<input type="hidden" id="some-place" name="some_place" value="classified">
<div class="col-sm-14">
<textarea name='message' placeholder="Additional Info" class="form-control" rows="7" style="border:1px solid #fff";></textarea></div>
<input class="form-control" type="text" name="name" style="border:1px solid #fff"; placeholder="Name">
<input class="form-control" type="email" name="email" style="border:1px solid #fff"; placeholder="Email" required>
<input class="btnbtn-action" type='submit' value="Send" onclick="return UpdateTheHiddenField()" >
</div>
</div>

I look forward to any assistance

[Erik_J]

@ChrisjChrisj

…find out how this Form checks for a proper email address:

The input has the attribute “required” put in in order to get the entered value checked to look like an email address when you submit because that is the type of the input.