How to overwrite / extent wordpress function is_email

is_email is a native WP function. As you can see here (https://core.trac.wordpress.org/browser/tags/5.0.3/src/wp-includes/formatting.php#L2886) it allows you to modify it with filter is_email.

So yes – anytime the function is_email will be called, it will run, and at the end it will run your filters allowing you to modify the result.

But… The filter should take 3 params:

$is_email (bool) Whether the email address has passed the is_email()
checks. Default false.

$email (string) The email address being checked.

$context (string) Context under which the email was tested.

function my_email_validation( $is_email, $email, $context )
{
    if ( <MY CONDITION> ) {  // check if it is an email
        return true;
    }

    return false;
}
add_filter( 'is_email', 'my_email_validation', 99, 3 );

So your code doesn’t work, because you use this filter incorrectly (you register it as it should take 4 params and your filter function takes only one, and so on).