The question remains: are we any safer?
The U.S. Postal Service's Tom Day helped design the system that now tests for anthrax at all 280 mail processing centers across the country. He gave CBS News correspondent Bianca Solorzano a tour of the John K. Rafferty Hamilton Post Office Building.
At least four of the anthrax letters came through this mail sorting center in New Jersey. It took four years to clean it up.
"This was the first spot where the anthrax was coming out of the envelopes," Day said, pointing to a mail sorting machine.
There has been a tunnel-like addition to the machine where letters collected from mail boxes are checked for anthrax.
"If anything is escaping from an envelope at this point, we're collecting it and pulling it out through a system right here," Day said. "That, then, goes to this box which is the self contained detection system."
The system's cost: $150 million per year.
So, after all the improvements, is our mail safe?
"I would definitely say the mail in this country is safe," Day said. "Can I give a 100 percent guarantee? The answer is 'no.'"
That's because the system doesn't check packages. It's also still a year away from screening large envelopes and testing for anything other than anthrax.
Then there are questions about private shippers who handle more than 21 million packages a day. They're tight-lipped about their security.
FedEx told CBS News: "We have systems in place to protect our employees and the safe transfer of packages to our customers."
UPS wouldn't even tell CBS News if they screen their cargo, saying "We do not talk about security measures because it would be counter productive to what we do."
Tom Ingelsby, who is Deputy Director of the Center for Biosecurity at the University of Pittsburgh Medical Center, said "This is a lot more complicated and will require a lot more time and investment than was anticipated back when this happened in 2001."
Bioterrorism experts warn detection holes are just part of the problem.
In an area where delays can cost lives, there is no stockpile of anthrax test kits that would quickly confirm suspected cases and allow for immediate treatment.
As for the anthrax vaccine, the government has ordered enough for 10 percent of the U.S. population. But that order won't be filled for at least three more years.