There were no letters sent before 9/11. I'd agree that a left wingnut might be nearly as likely as a right wingnut, but the main body of evidence points to a domestic wingnut.
OBL had 9/11 planned over years. With the resources at OBL's hands, he would have found a way to inflict tons of the stuff on America rather than a pathetic few ounces. The evidence points to a very determined individual with limited resources.
You still don't get it. The anthrax comes from Saddam. Saddam is the architect of 9/11. OBL provided recruits -- kamikaze pilots -- and not much more than that. The anthrax letters are designed to deter Bush from retaliating against Iraq.
This is how warfare works: you commit acts of violence on your opponent, and threaten something even worse, gambling that your opponent will back down. Only the means are unconventional -- the veil of terrorism, and the use of biological weapons. The underlying strategy is as old as history.
There are so many reasons single-piece First-Class Mail can get screwed up I don't have room to discuss them all. So, do not confuse date of cancellation with date of mailing!
Can you point to any evidence for this at all?
All the evidence so far points to 9/11-related international activity. Most of the evidence is circumstantial, but that's where it points. There's no evidence, circumstantial or otherwise, that points to a domestic nut.
Now, you'll find articles reporting that somebody says it's probably domestic, but those articles never contain any actual evidence.
I should add that it wouldn't surprise me greatly if there was some domestic involvement -- for example, the anthrax might have been stolen from a U.S. lab by an American and then sold to a terrorist or foreign agent. But, based on the evidence we've seen, the cause and rationale behind the anthrax attack will be found abroad.