Also, could any institutional recipients have just thrown away mail that looked like it came from kooks without even bothering to open it?
Then there is the USPS Brentwood facility, the main post office for the United States federal government!
One obscure facility you have to think about is the building where mail to the US Congress and the Executive is checked for various chemical signatures. The attacker had to have planned for some of the anthrax attack letters to go through that operation. If that building were disabled through anthrax contamination then there would be no easy way to interdict letter or parcel mail bombs being sent to high government officials and members of Congress.
Regarding Palm Beach airport, a considerable percentage of America's wealthy and powerful pass through there every day. Their stuff comes in through the terminal handling operations. Even Teddy Kennedy used to travel through this one in between snorts!
I'm just guessing that the fellows who masterminded this one had prior experience in their own country with a contaminated air terminal or postal operating center.
Google has some good references to airport decontamination ~ several involve Red Chinese controlled Hong Kong.