(This is a rough draft. Please correct spelling errors and stupid sentences. English isn't my native tongue, so bear with me.)
Having observed computer illiterates struggling with organizing vast amounts of e-mail using folders, I realized that a new approach was necessary. I surely haven't invented any of the concepts on this page, I just present them to you.
As has been said many times in the recent past, the human mind isn't fit for using hierarchical archive systems. In the physical world, we often have to, but that's not true in the digital realm. Therefore...
Message Categories resemble the Saved Search Folders, except they go even further in terms of dynamism and flexibility. They're meant as a completely alternative to hierarchical folders.
A Message Category is a named set of message filters. These filters can include sender, date, subject, receiver, content, importance, number and kinds of attachments, etc. Furthermore, there are two boolean message properties that can be used in a filter; whether the message is marked as junk and whether it has been marked as deleted.
The "Junk" category would then have a single filter that matches all messages marked as junk. The "Trash" or "Deleted" category would contain all messages marked as deleted. All other categories would by default exclude these messages.
Tags could also be used to categorize messages. For instance, the category "Firefox" could contain a filter that matched all messages tagged "firefox".
Categories can be nested, in the sense that a subcategory can contain a subset of its parent's messages. A subcategory can never include messages not included by its parent. If the "Firefox" category had a subcategory labeled "Bon Echo", that included messages tagged "bon echo", it would contain only messages tagged both "firefox" and "bon echo".
Categories can in theory be nested infinitely, each subcategory excluding more messages.
By default, messages included in a subcategory will not also be included in the parent category. Therefore, "Firefox" would not include messages tagged "bon echo". This could be changed by setting an option in "Bon Echo"'s properties dialog, e.g. "Display matched messages in the parent category?".
Message Category Pane
The Message Category Pane would take the place of the current Folder Pane. It would display a nested list of categories.
This is just a rough draft of what default categories the Message Category Pane may include.
.---------------------. | @ Messages | | @ Received | | @ Unread | | @ Important | | @ Pending | | @ Drafts | | @ Sent | | @ Junk | | @ Trash | '---------------------'
Users may also explicitly categorize their messages by either dragging the selected messages to a category, or use some other sort of UI (TBD). A message can be in an arbitrary number of categories.
The user can execute commands on all messages in a category - this includes "delete", "mark as junk", "mark as read", "tag", etc.
If the user, while in the "Bon Echo" category, searced for messages whose bodies included the word "places", he could save that search as a category ("Places"). The new category would in this case have a filter that matched all messages whose bodies contained the word "places" (duh!).
A saved search will by default be a subcategory of the category in which the search took place. Thus, the Places category would be a subcategory of "Bon Echo". This can be manually overridden.
Subcategories can be moved from one category to another - the only change is that a different set of filters will be applied before those of the subcategory itself.