[John H K]

one would think that authorities (and the public at large) would be on the look out for Middle Eastern looking types dropping off a suspicious letter/package at mailboxes/post offices

None of the anthrax letters were "suspicious looking" to anyone more than 1 foot away from them, and how can one be on the "lookout" for a routine-looking letter dropped in a street mailbox, of which there are tens of thousands in a single state, by ANYBODY?

[anymouse]

It was determined that all of the Anthrax letters were mailed from mailboxes in a particular region (zip code) of W. NJ. Even without more descriptive details you bet every postal worker and cop in the area was watching every mailbox in that area with some suspicion. Maybe not very effective, but human nature makes people warry of bad past experiences (don't touch a hot stove cause it hurts when you get burned.)