A wordpress mu-plugin that allows you to create users with email-addresses as usernames in multisite

allow-email-as-username.php

<?php
/**
 * Plugin name: Allow email as WordPress Network/Multisite username
 * Description: A wordpress mu-plugin that allows you to create users with email-addresses as usernames in multisite
 * Version: 1.0
 * Author: @anttiviljami
 * License: GPLv3
 */

add_filter( 'wpmu_validate_user_signup', '_signup_allow_email_as_username' );
function _signup_allow_email_as_username($result) {
  if (! is_wp_error($result['errors'])) {
    return $result;
  }

  $username = $result['user_name'];

  // Copy any error messages that have not been overridden
  $new_errors = new WP_Error();

  $errors = $result['errors'];
  $codes = $errors->get_error_codes();

  foreach ($codes as $code) {
    $messages = $errors->get_error_messages($code);

    if ($code == 'user_name') {
      foreach ($messages as $message) {
        if ($message == __('Only lowercase letters (a-z) and numbers are allowed.')) {
          if (!is_email($username)) {
            $new_errors->add($code, $message);
          }
        } else {
          // Restore other username errors
          $new_errors->add($code, $message);
        }
      }
    }
    else {
      // Restore any other errors
      foreach ($messages as $message) {
        $new_errors->add($code, $message);
      }
    }
  }

  $result['errors'] = $new_errors;
  return $result;
}