I thought that quite curious, and could be a clue in some tie, like a child, to the perpetrator, although I guess that is too obvious.
Yes it is. Notice the other letters to the news outfits are copies with no return @. The folks there don't care about return @'s. The letters also tell them to take penicillin. The perps not interested in a kill, just a slime for effect. Notice those letters had their anthrax cut with something, it was lower grade/same bug.
The letter to Daschle was sent from the "4th grade", the perp wanted to give it the best chance of hitting the target. He was posing as "the children" to the senator. It was uncut, high quality, potent stuff. He was intending harm.
These letters weren't sent by jihadists. The first priority with them is the big kill, big time blood and guts massacre. These letters were sent by a single authoritarian, masquerading as a jihadist. He was set to go before 9-11, and the jihadist action gave him the opportunity to act to forward his agenda.