The mailing tray into which it was tossed, or maybe a piece of mail in that tray, was then contamined by one of the primary anthrax letters.
Remember, the secondary contamination of postal equipment was more deadly than any primary contact with the letters. Most of the letters got trapped. Only the stationary equipment that got contaminated was stopped in place. Hundreds of thousands of letter and flat trays that were potentially contaminated managed to get back out into the postal system before the anthrax attack was known to have happened.
"IF the anthrax arrived at AMI other than the US mail, why then would Stephanie Daily and Ernesto Blanco be the only two other persons who tested positive."
Hard to argue with that. And the concentration of spores found in the mailroom area too.
Ed has interesting idea about spread by vacuum, but how about shoes? And did successive vacuuming clean up some of the spread, and were all the floors cleaned as much? And what were there surfaces, carpet, flooring, etc. Many variables.
My idea on a possible "two letter" solution depends on the Ms. Dailey. It is reported that it was her duty to open mail addressed to the National Enquirer. OK, who opens mail addressed to the other publications? Isn't it plausible that the perp went two letters, one to the National Enquirer, one to say, The Sun, and the Sun one was opened first, the Enquirer later when Dailey returned from vacation?
As to the mailer (not necessarily producer) being a sophisticated scientist, wouldn't that kind of person more likely target The New York Times and Newsweek rather than the New York Post and National Enquirer?
Blanco worked the mailroom. Stephanie opened some letters. But Bob Stevens had nothing to do with the mailroom. It's Stevens that the letter theory doesn't fit.
Sure, the anthrax *could* have been sent via a letter (though no such letter was ever found), but it could also have come in through tainted cash.
Cash follows funny paths that could easily have crossed by all three AMI employees.