The concept of having 43 folders is not about having folders... it's about mailing yourself something in the future. In the book, Allen mentions that some people use a calendar for the folders. The calendar, or a with the task management software, a due date or appearing date can often function as the 43 folders.
The main principle for the 43 folders is how do you deal with physical items that you want to be reminded of in the future.
In the electronic world, I don't get that many emails of something that I need to look at in the future that I can't just add to my task management software to be reminded of in the future. It's the beauty of digital.
If you want a tickler file for mac mail:
http://www.hawkwings.net/2005/12/01/...-tickler-file/