Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 28377)
+++ wp-includes/ms-functions.php	(working copy)
@@ -828,8 +828,10 @@
 
 	$activate_url = esc_url($activate_url);
 	$admin_email = get_site_option( 'admin_email' );
-	if ( $admin_email == '' )
-		$admin_email = 'support@' . $_SERVER['SERVER_NAME'];
+	if ( $admin_email == '' ) {
+		global $current_site;
+		$admin_email = 'support@' . $current_site->domain;
+	}
 	$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";
 	$message = sprintf(
@@ -921,8 +923,10 @@
 
 	// Send email with activation link.
 	$admin_email = get_site_option( 'admin_email' );
-	if ( $admin_email == '' )
-		$admin_email = 'support@' . $_SERVER['SERVER_NAME'];
+	if ( $admin_email == '' ) {
+		global $current_site;
+		$admin_email = 'support@' . $current_site->domain;
+	}
 	$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";
 	$message = sprintf(
@@ -1504,7 +1508,7 @@
 	$admin_email = get_site_option( 'admin_email' );
 
 	if ( $admin_email == '' )
-		$admin_email = 'support@' . $_SERVER['SERVER_NAME'];
+		$admin_email = 'support@' . $current_site->domain;
 
 	$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";
@@ -1582,7 +1586,7 @@
 	$admin_email = get_site_option( 'admin_email' );
 
 	if ( $admin_email == '' )
-		$admin_email = 'support@' . $_SERVER['SERVER_NAME'];
+		$admin_email = 'support@' . $current_site->domain;
 
 	$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";
Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 28377)
+++ wp-includes/pluggable.php	(working copy)
@@ -365,7 +365,12 @@
 
 	if ( !isset( $from_email ) ) {
 		// Get the site domain and get rid of www.
-		$sitename = strtolower( $_SERVER['SERVER_NAME'] );
+		if ( is_multisite() ) {
+			global $current_blog;
+			$sitename = $current_blog->domain;
+		} else {
+			$sitename = parse_url( home_url(), PHP_URL_HOST );
+		}
 		if ( substr( $sitename, 0, 4 ) == 'www.' ) {
 			$sitename = substr( $sitename, 4 );
 		}
@@ -1390,7 +1395,13 @@
 		$notify_message .= sprintf( __('Spam it: %s'), admin_url("comment.php?action=spam&c=$comment_id") ) . "\r\n";
 	}
 
-	$wp_email = 'wordpress@' . preg_replace('#^www\.#', '', strtolower($_SERVER['SERVER_NAME']));
+	if ( is_multisite() ) {
+		global $current_blog;
+		$sitename = $current_blog->domain;
+	} else {
+		$sitename = parse_url( home_url(), PHP_URL_HOST );
+	}
+	$wp_email = 'wordpress@' . preg_replace( '#^www\.#', '', $sitename );
 
 	if ( '' == $comment->comment_author ) {
 		$from = "From: \"$blogname\" <$wp_email>";