You can't track the email directly, but you can setup a workflow rule to check a change in # of Email on the work item, and notify the manager. However, I don't think you can tell between outgoing & incoming emails or whether TrackIt was the recipient this way.
I can suggest better alternatives to keep track of work item correspondence:
- Use posts & discussions instead of emails. Posts and replies are also sent by email so the manager will be notified + you'll have the correspondence tracked comfortably in the Discussions of the work item. I believe that covers your scenario, as one can reply to a post by email (and their reply will appear as one in the feed).
- Tracking incoming emails is exactly what InterAct is designed for. If you format the emails so replies are sent to an InterAct mailbox, you can fully control the processing of the replies.
Hope this helps,