If i patent a device that puts mail in peoples mail boxes and the someone else tries to patent a different way of doing the same thing, and one of my approved claims is for devices that put mail in mailboxes, would that invalidate the new patent?
A patent is as broad as its allowed claims are. In your example, if a patent gets a claim for "devices that put mail in mailboxes". Then if someone figures out a different implementation they still infringe. However, I think it is very unlikely for your patent to get such a claim. Generally speaking there needs to be an inventive step to get a patent granted. Usually this is a specific implementation with something novel and non-obvious over prior art. Yes, there are bad patents out there with overly broad claims. This in no way implies all patents are ridiculously broad. Indeed my experience is that the USPTO sometimes goes through extraordinary lengths to conjure up unlikely combinations of prior art to reject patent applications.