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.
The very first indicator is the hand writing. Unless someone has extensive training in hand writing analysis(which of course, could not be totally eliminated as a possibility) how someone learns language is very difficult to mask. The typical American printing is much different than the typical arabic printing. American printing tends to be block letters (such as were contained in the letters). Arabic printing tends to have lots of serifs (curley letters). American printing tends to have a forward slant (such as were contained in the letters). Arabic printing tends to have a backward slant, because they learn writing from right to left instead of left to right.
A very strong indicator that points to a deranged individual rather than a state or a well financed organization is simply the total amount of anthrax. The entire amount added up to no more than a few ounces. Iraq has tons of the stuff, they more likely would have put it in thousands of pieces of mail instead of just a few. It appears to be the product of a very bright individual working with very limited resources.
Another indicator is the timing. The earliest date of cancelation was about a week after 9/11. This points to someone who did not know 9/11 was going to happen, but saw it as an opportunity to hide their own tracks.