All the letters are said to be photocopies. (It's obvious that at least one of each pair is a photocopy, but I've that, in fact, they didn't mail any original.)
They don't have the low-resolution appearance typically seen in faxes.
Atta and al Shehhi were in Spain in early July. They were both in Hamburg at some time in the summer. When is the latest we know any of the hijackers were in Europe? I think I've read Binalshibh was in Madrid days before 9/11, but he of course was not one of the hijackers, and never succeeded in getting a visa to enter this country.
What means was there to transport photocopies made in Europe to the U.S. days before 9/11? (Remember, in one of his videotapes last year, bin Laden said he learned about the date of the planned attacks the Thursday before, i.e., Sept. 6th, which just happens to be the day the short selling of the stocks started.) Could they have been transported by Iraqi diplomatic pouch?