Or is it to make people afraid, to disrupt the mail system, office buildings, investigators, testing labs, etc? It seems to me that the latter is the case, and in those terms the efforts of whoever is responsible have been largely successful. I would think that it wouldn't really be all that hard to distribute the poweder they have produced in such a way as to expose a lot more people, and probably less obtrusively... standing on a high building and thowing it out a window over a crowded area, riding a bicycle through a crowd with an open container blowing in the wind, etc, etc. Powder in a letter seems quite ineffective if your goal is to infect the maximum number of people.

There has been a lot of press about how hard it is to make really good weaponized anthrax powder, but these people probably would be happy with half-assed material. If the infection rate was 20% instead of 95% they would still consider themselves successful.