AARONHARNLY The packets you're seeing are from the *optional* check for new versions, which also optionally sends anonymous configuration information (what version of Mail, your processor speed, and screen size). The first time you run Mail with Letterbox installed, it asks whether you want to enable the version check, and whether to send the anonymous information.
You can disable it by opening the Letterbox preferences, going to the "Updates" tab, and unchecking the box.
Hope that addresses your concerns!