diff --git a/wp-includes/ms-functions.php b/wp-includes/ms-functions.php
index 583d4a213c..bfb2b157e1 100644
--- a/wp-includes/ms-functions.php
+++ b/wp-includes/ms-functions.php
@@ -952,7 +952,7 @@ function wpmu_signup_blog_notification( $domain, $path, $title, $user_login, $us
 	$admin_email = get_site_option( 'admin_email' );
 
 	if ( '' === $admin_email ) {
-		$admin_email = 'support@' . $_SERVER['SERVER_NAME'];
+		$admin_email = 'support@' . wp_parse_url( network_home_url(), PHP_URL_HOST );
 	}
 
 	$from_name       = ( '' !== get_site_option( 'site_name' ) ) ? esc_html( get_site_option( 'site_name' ) ) : 'WordPress';
@@ -1079,7 +1079,7 @@ function wpmu_signup_user_notification( $user_login, $user_email, $key, $meta =
 	$admin_email = get_site_option( 'admin_email' );
 
 	if ( '' === $admin_email ) {
-		$admin_email = 'support@' . $_SERVER['SERVER_NAME'];
+		$admin_email = 'support@' . wp_parse_url( network_home_url(), PHP_URL_HOST );
 	}
 
 	$from_name       = ( '' !== get_site_option( 'site_name' ) ) ? esc_html( get_site_option( 'site_name' ) ) : 'WordPress';
@@ -1663,7 +1663,7 @@ We hope you enjoy your new site. Thanks!
 	$admin_email = get_site_option( 'admin_email' );
 
 	if ( '' === $admin_email ) {
-		$admin_email = 'support@' . $_SERVER['SERVER_NAME'];
+		$admin_email = 'support@' . wp_parse_url( network_home_url(), PHP_URL_HOST );
 	}
 
 	$from_name       = ( '' !== get_site_option( 'site_name' ) ) ? esc_html( get_site_option( 'site_name' ) ) : 'WordPress';
@@ -1755,7 +1755,7 @@ function wpmu_welcome_user_notification( $user_id, $password, $meta = array() )
 	$admin_email = get_site_option( 'admin_email' );
 
 	if ( '' === $admin_email ) {
-		$admin_email = 'support@' . $_SERVER['SERVER_NAME'];
+		$admin_email = 'support@' . wp_parse_url( network_home_url(), PHP_URL_HOST );
 	}
 
 	$from_name       = ( '' !== get_site_option( 'site_name' ) ) ? esc_html( get_site_option( 'site_name' ) ) : 'WordPress';
diff --git a/wp-includes/pluggable.php b/wp-includes/pluggable.php
index 0af4eb42d2..c614ac0ed6 100644
--- a/wp-includes/pluggable.php
+++ b/wp-includes/pluggable.php
@@ -330,7 +330,7 @@ if ( ! function_exists( 'wp_mail' ) ) :
 		 */
 		if ( ! isset( $from_email ) ) {
 			// Get the site domain and get rid of www.
-			$sitename = strtolower( $_SERVER['SERVER_NAME'] );
+			$sitename = wp_parse_url( network_home_url(), PHP_URL_HOST );
 			if ( 'www.' === substr( $sitename, 0, 4 ) ) {
 				$sitename = substr( $sitename, 4 );
 			}
@@ -1650,7 +1650,7 @@ if ( ! function_exists( 'wp_notify_postauthor' ) ) :
 			$notify_message .= sprintf( __( 'Spam it: %s' ), admin_url( "comment.php?action=spam&c={$comment->comment_ID}#wpbody-content" ) ) . "\r\n";
 		}
 
-		$wp_email = 'wordpress@' . preg_replace( '#^www\.#', '', strtolower( $_SERVER['SERVER_NAME'] ) );
+		$wp_email = 'wordpress@' . preg_replace( '#^www\.#', '', wp_parse_url( network_home_url(), PHP_URL_HOST ) );
 
 		if ( '' === $comment->comment_author ) {
 			$from = "From: \"$blogname\" <$wp_email>";
@@ -2831,4 +2831,3 @@ if ( ! function_exists( 'wp_text_diff' ) ) :
 		return $r;
 	}
 endif;
-
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 45a79f799e..82d748cb13 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.5-beta3-48600';
+$wp_version = '5.5-beta3-48601';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.