Index: src/wp-includes/ms-functions.php
===================================================================
--- src/wp-includes/ms-functions.php	(revision 47544)
+++ src/wp-includes/ms-functions.php	(working copy)
@@ -946,7 +946,7 @@
 	$activate_url = esc_url( $activate_url );
 	$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' ) == '' ? 'WordPress' : esc_html( get_site_option( 'site_name' ) );
 	$message_headers = "From: \"{$from_name}\" <{$admin_email}>\n" . 'Content-Type: text/plain; charset="' . get_option( 'blog_charset' ) . "\"\n";
@@ -1071,7 +1071,7 @@
 	// Send email with activation link.
 	$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' ) == '' ? 'WordPress' : esc_html( get_site_option( 'site_name' ) );
 	$message_headers = "From: \"{$from_name}\" <{$admin_email}>\n" . 'Content-Type: text/plain; charset="' . get_option( 'blog_charset' ) . "\"\n";
@@ -1635,7 +1635,7 @@
 	$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' ) == '' ? 'WordPress' : esc_html( get_site_option( 'site_name' ) );
@@ -1727,7 +1727,7 @@
 	$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' ) == '' ? 'WordPress' : esc_html( get_site_option( 'site_name' ) );
Index: src/wp-includes/pluggable.php
===================================================================
--- src/wp-includes/pluggable.php	(revision 47544)
+++ src/wp-includes/pluggable.php	(working copy)
@@ -328,7 +328,7 @@
 		 */
 		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 ( substr( $sitename, 0, 4 ) == 'www.' ) {
 				$sitename = substr( $sitename, 4 );
 			}
@@ -1640,7 +1640,7 @@
 			$notify_message .= sprintf( __( 'Spam it: %s' ), admin_url( "comment.php?action=spam&c={$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>";