First of all, it doesn't even need to get inside of other envelopes to infect people through their mail. All it needs to do is get all over the outside of their envelopes, and they'll still get covered with it when they pick up their mail. And since sufficient anthrax has been escaping the original envelopes to infect postal workers, it's been utterly, patently obvious from the beginning that equal amounts of anthrax could be expected to get all over the random letters that happened to be stacked directly above and below the anthrax-filled ones.
Secondly, I don't know what the anthrax-filled letters were sealed with exactly, but most envelopes that arrive in my mailbox have substantial airholes around the seal at the top, plus things like bent corners with pinhole-sized worn spots on the edges. I doubt that a single envelope that's arrived in my mailbox in the past year has been sufficiently "airtight" (ha ha) to keep even a large sized ladybug from crawling in and out if it was determined enough to make the effort - let alone a few tiny bacteria.
I refuse to believe that our nation's top "experts" could possibly be so completely totally stupid as not to have figured this out. They've been absolutely 100% knowingly lying through their teeth for weeks now, for the simple reason that they're terrified of the panic that could result from formally acknowledging to the American public that yes, indeed, the personal letters you get from your dearest friends in your very own mailbox could indeed be completely covered in anthrax.
And yet, it's not as though there's really cause for panic, either. We've all still got a much better chance of winning a million dollars in the lottery than of being infected with anthrax. But there is that tiny little chance, and I wish they'd just hurry up and admit it and stop playing stupid by trying to deny something so patently obvious.