diff --git a/wp-includes/ms-functions.php b/wp-includes/ms-functions.php
index 93662fa923..cc797a7bd0 100644
--- a/wp-includes/ms-functions.php
+++ b/wp-includes/ms-functions.php
@@ -497,8 +497,9 @@ function wpmu_validate_user_signup( $user_name, $user_email ) {
 
 	$limited_email_domains = get_site_option( 'limited_email_domains' );
 	if ( is_array( $limited_email_domains ) && ! empty( $limited_email_domains ) ) {
-		$emaildomain = substr( $user_email, 1 + strpos( $user_email, '@' ) );
-		if ( ! in_array( $emaildomain, $limited_email_domains ) ) {
+		$limited_email_domains = array_map( 'strtolower', $limited_email_domains );
+		$emaildomain = strtolower( substr( $user_email, 1 + strpos( $user_email, '@' ) ) );
+		if ( ! in_array( $emaildomain, $limited_email_domains, true ) ) {
 			$errors->add( 'user_email', __( 'Sorry, that email address is not allowed!' ) );
 		}
 	}
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 68e3dad7eb..2a7ce8cdbe 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-alpha-42857';
+$wp_version = '5.0-alpha-42858';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.