There’s a rake task for sorting posts within a topic, but I do not recommend using it on more than a couple of topics as long as you still have a chance to fix the order within the import script.
rake posts:reorder_posts[topic_id]
Out of curiosity: Did you follow Importing mailing lists (mbox, Listserv, Google Groups, emails, ...)?
Would you mind sending me a PM with the source of all the emails in that topic? I’d like to take a look why the import script isn’t sorting the emails correctly.