This is a stupid question but as I'm not a biologist, I'll ask it anyway and risk looking foolish...
Would it help to microwave your mail if you don't know where it came from and you're sure it doesn't contain an objects? i.e. it's not a CD you've ordered from Amazon. :)
Do anthrax spores get cooked enough by microwaves to be killed, or are they too dry in spore form to be nuked?
Of course, if we start mass nuking our mail, the terrorists might mail us something that would explode when nuked... still, for now it's a question to ask.
Spores tend to be pretty rugged. One of the reasons anthrax is such an ideal weapon is that its spores are rugged enough to tolerate being used on missiles and bombs, which means extremes of temperature, I supose. I think you would need to microwave it pretty intensely to kill it. Microwave radiation is non-ionizing so it kills things by heating the entire things, which means you need to use enough heat to denature enough proteins to stop metabolism. Using radio isotopes to irradiate it would be a different matter. That might be much more effective and would do less damage to the mail than a microwave oven. The right kinds of radiation are ionizing, which causes very reactive free radicals to be formed in the cell. It doesn't take very many of these free radicals to damage the DNA beyond usefulness, usually by breaking it. Some organisms are radio resistant, which means they have hyper-active DNA repair systems and multiple copies of important enzymes. This comes at a cost of needing more energy to run these systems and to build more DNA so the organisms are at a disadvantage compared to non-radio-resistant organisms in non-radio environments. If you wanted to build a real weapon-type anthrax you could do stuff like grow it with increasing levels of antiobiotics, so it would get resistant to them, and then you could also expose it to heat, radiation, whatever. I'm sure the superpowers have done that kind of stuff. The principles behind this are simple, but the practice is probably quite a challenge. It would be interesting to know if the anthrax that is showing up in the mail is the natural stuff or some kind of weapon-grade stuff. The safest thing to do is to stick to email. In a yurt. In Antarctica.