The postal system is particularly vulnerable to poisons because its main defenses are all aimed at killing or detecting harmful living organisms, like anthrax, which is a bacterium. Irradiation machines, which sterilize all first-class mail bound for Washington government offices, work by disrupting an organism's DNA. They have no effect on poisons, which are simply molecules that happen to have devastating effects on human physiology.
Moreover, though the Postal Service is installing air sampling systems to test for anthrax spores around sorting machines at 280 regional mail hubs, these systems, to be ready starting in March, will not initially be able to test for poisons or other harmful substances.
After the anthrax scare I was pretty sure the postal service was finished. Others agreed, Adobe got a boost in its share price because its Acrobat technology is an alternative to mailed documents.
Didn't happen. Turned out there really aren't that many competent whackos out there. I'd have to guess that only a half dozen Americans combine both reasonable intelligence and insane malevolence (see the Unabomber, a victim of paranoid schizophrenia). Even al Qaeda, I suspect, is having a very hard time finding competent fanatics -- not the least because a large number of their "best" are dead (including the 9/11 hijackers or in captivity).
The Ricin could be al Qaeda, but even they would know that no-one in the US Government opens their own mail. I suspect it's another paranoid schizophrenic, and that's probably who the FBI are looking for. On the other hand, Richard (the shoe) Reid was both was probably a threefer -- developmental delay, paranoid schizophrenia, and al Qaeda.