Note the heavy blobs at the beginning & ending of each letter. That's a sign of "pausing" - perhaps copying it from a list of addresses, and not familiar with the "characters". Possibly someone who is "foreign". The deliberate slowness indicated in the writing may be indicative of someone not well educated. Spacing is mechanical, but not well thought out...note that in several places the characters go out to the edge, and the letters start to squash together.
It may have been licked to seal and the stamp too...and that gives a sample of DNA in the saliva for cross matching when they arrest the perp.
Maybe, but the thing is, it's also easy to read into it whatever you want to read into it. I notice that those who are already convinced that it's from a Middle Easterner sees what they want to see. I'm on the fence, so I'm seeing the possibility that it could be, but darn, I just have to say that this particular handwriting looks like it came from someone over 65 or 70 with arthritis. I've read a lot of European handwriting, and some cards from Middle Easterners too. This really doesn't look like typical ME handwriting--but also note, just like U.S. handwriting, it can vary a whole lot from person to person. Maybe it's from one of the terrorist's grandparents??
Now also folks, please consider: this particular envelope did not test positive for anthrax, and might be a hoax! We might be spinning our wheels on the wrong envelope, and the wrong handwriting sample. When will they release the contents of the other "threatening letters"--and photographs of the envelopes? Now those might be something to sink our teeth into!