diff --git a/wp-admin/includes/upgrade.php b/wp-admin/includes/upgrade.php
index 7042f2c2a..dbd52bdd1 100644
--- a/wp-admin/includes/upgrade.php
+++ b/wp-admin/includes/upgrade.php
@@ -159,7 +159,6 @@ function wp_new_blog_notification($blog_title, $blog_url, $user_id, $password) {
 	$user = new WP_User($user_id);
 	$email = $user->user_email;
 	$name = $user->user_login;
-	$message_headers = 'From: "' . $blog_title . '" <wordpress@' . $_SERVER['SERVER_NAME'] . '>';
 	$message = sprintf(__("Your new WordPress blog has been successfully set up at:
 
 %1\$s
@@ -175,7 +174,7 @@ We hope you enjoy your new blog. Thanks!
 http://wordpress.org/
 "), $blog_url, $name, $password);
 
-	@wp_mail($email, __('New WordPress Blog'), $message, $message_headers);
+	@wp_mail($email, __('New WordPress Blog'), $message);
 }
 endif;
 
diff --git a/wp-includes/pluggable.php b/wp-includes/pluggable.php
index 84511f33a..fc6fc8ff8 100644
--- a/wp-includes/pluggable.php
+++ b/wp-includes/pluggable.php
@@ -328,7 +328,13 @@ function wp_mail( $to, $subject, $message, $headers = '', $attachments = array()
 		$from_name = 'WordPress';
 	}
 
-	// If we don't have an email from the input headers
+	/* If we don't have an email from the input headers default to wordpress@$sitename
+	 * Some hosts will block outgoing mail from this address if it doesn't exist but
+	 * there's no easy alternative. Defaulting to admin_email might appear to be another 
+	 * option but some hosts may refuse to relay mail from an unknown domain. See
+	 * http://trac.wordpress.org/ticket/5007.
+	 */
+	
 	if ( !isset( $from_email ) ) {
 		// Get the site domain and get rid of www.
 		$sitename = strtolower( $_SERVER['SERVER_NAME'] );
@@ -339,7 +345,7 @@ function wp_mail( $to, $subject, $message, $headers = '', $attachments = array()
 		$from_email = 'wordpress@' . $sitename;
 	}
 
-	// Set the from name and email
+	// 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 );