commit a923fbb07aaeb1eb53f94343062f36561373abc1
Author: Robert Hailey <rhailey@allogy.com>
Date:   Mon Apr 1 13:19:05 2013 -0500

    f: add in-reply-to field (fixes #3228)

diff --git a/app/models/mailer.rb b/app/models/mailer.rb
index b91ea84..c973503 100644
--- a/app/models/mailer.rb
+++ b/app/models/mailer.rb
@@ -347,6 +347,7 @@ class Mailer < ActionMailer::Base
     end
     if @references_objects
       mail.references = @references_objects.collect {|o| self.class.message_id_for(o)}
+	  mail.in_reply_to = mail.references
     end
 
     # Log errors when raise_delivery_errors is set to false, Rails does not