Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 25241)
+++ wp-includes/ms-functions.php	(working copy)
@@ -723,7 +723,7 @@
 	$activate_url = esc_url($activate_url);
 	$admin_email = get_site_option( 'admin_email' );
 	if ( $admin_email == '' )
-		$admin_email = 'support@' . $_SERVER['SERVER_NAME'];
+		$admin_email = 'support@' . $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(
@@ -770,6 +770,8 @@
  * @return bool
  */
 function wpmu_signup_user_notification( $user, $user_email, $key, $meta = array() ) {
+	global $current_site;
+	
 	if ( !apply_filters('wpmu_signup_user_notification', $user, $user_email, $key, $meta) )
 		return false;
 
@@ -776,7 +778,7 @@
 	// Send email with activation link.
 	$admin_email = get_site_option( 'admin_email' );
 	if ( $admin_email == '' )
-		$admin_email = 'support@' . $_SERVER['SERVER_NAME'];
+		$admin_email = 'support@' . $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(
@@ -1236,7 +1238,7 @@
 	$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";
@@ -1284,7 +1286,7 @@
 	$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}\" <{$admin_email}>\n" . "Content-Type: text/plain; charset=\"" . get_option('blog_charset') . "\"\n";
Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 25241)
+++ wp-includes/pluggable.php	(working copy)
@@ -218,7 +218,7 @@
 	if ( !is_array($attachments) )
 		$attachments = explode( "\n", str_replace( "\r\n", "\n", $attachments ) );
 
-	global $phpmailer;
+	global $phpmailer, $current_site;
 
 	// (Re)create it, if it's gone missing
 	if ( !is_object( $phpmailer ) || !is_a( $phpmailer, 'PHPMailer' ) ) {
@@ -326,16 +326,9 @@
 	 * http://trac.wordpress.org/ticket/5007.
 	 */
 
-	if ( !isset( $from_email ) ) {
-		// Get the site domain and get rid of www.
-		$sitename = strtolower( $_SERVER['SERVER_NAME'] );
-		if ( substr( $sitename, 0, 4 ) == 'www.' ) {
-			$sitename = substr( $sitename, 4 );
-		}
+	if ( !isset( $from_email ) )
+		$from_email = 'wordpress@' . $current_site->domain;
 
-		$from_email = 'wordpress@' . $sitename;
-	}
-
 	// Plugin authors can override the potentially troublesome default
 	$phpmailer->From     = apply_filters( 'wp_mail_from'     , $from_email );
 	$phpmailer->FromName = apply_filters( 'wp_mail_from_name', $from_name  );
@@ -1007,6 +1000,8 @@
  * @return bool False if user email does not exist. True on completion.
  */
 function wp_notify_postauthor( $comment_id, $comment_type = '' ) {
+	global $current_site;
+	
 	$comment = get_comment( $comment_id );
 	$post    = get_post( $comment->comment_post_ID );
 	$author  = get_userdata( $post->post_author );
@@ -1076,7 +1071,7 @@
 		$notify_message .= sprintf( __('Spam it: %s'), admin_url("comment.php?action=spam&c=$comment_id") ) . "\r\n";
 	}
 
-	$wp_email = 'wordpress@' . preg_replace('#^www\.#', '', strtolower($_SERVER['SERVER_NAME']));
+	$wp_email = 'wordpress@'  . $current_site->domain;
 
 	if ( '' == $comment->comment_author ) {
 		$from = "From: \"$blogname\" <$wp_email>";