Comment 8 for bug 744935

The way it is now, Delete turns into Undelete inside the Trash folder. The Archive button could behave in the same fashion and move messages back into the Inbox. That is the most common use case.

Indexing will, unlike now, allow arbitrary meta data such as "where did a message come from originally". That would be an enhancement, not a requirement, in this case, from my perspective. You can have a look at the relevant blueprint, it's not quite uptodate currently but should give you an idea, I need to fix it.