Rated 4 out of 5 stars
Its message tracking idea is very good, but it's very messed up and confuses the user. Its diagram is also placed in a tiny box which is hard to use and see. Additionally it approximately slows Thunderbird.
A good improvement can be moving the diagram to vertical style and combine it with something like conversations. You can put those diagrams vertically at the left or right side of the conversation messages for easy tracking.