In MailBox is chosen for a powerful user interface, which means that the library has to do all the work. Therefore, the MailBox library is quite large. The major advantages of this approach are: closely integrated components, with consistent documentation, tests which cover the parts together, more complex examples in the code.
The disadvantages: large library (over 140 packages) which is hard to grasp. Too much functionality for every application (no-one uses all)