1979 Russian anthrax release
<This was the result of a missing filter... 94 infections, 64 deaths. It seems clear that a wide, purposeful release of this grade of anthrax would be very serious.
The envelopes are just "play", in relative terms. I think anyone with the means to produce high grade weaponized anthrax would come up with a more effective means of spreading it.
Of course, as I mentioned earlier, these envelopes have caused a whole lot of trouble, even if they haven't caused very many cases of anthrax yet.