functions.php

/* Force email lowercase on WordPress registration
------------------------------------------------------------------ */
function filter_pre_user_email( $raw_user_email ) { 
    
    $raw_user_email = strtolower($raw_user_email);
    
    return $raw_user_email; 
}; 
         
// add the filter 
add_filter( 'pre_user_email', 'filter_pre_user_email', 10, 1 );