Though my last bacteriology class was almost sixty years ago, I still remember some of the fundamentals.
A microwave is not a good place to sterilize much of anything. The only places that get hot are those with fluid enough to absorb enough of the radiant energy.Even with an autoclave, it takes roughly half an hour to disinfect things. What's worse is that the microwave would be very liable to create dangerous aerosols of infectious agents. Only the very heat resistant spores of infectious agents would be used by a terrorist.
Not until there is an official warning would I worry about mass mailings to individuals. I'll bet that the few mailings so far have been made by good old USA kooks.