This is something that just occurred to me and may have occurred to you guys as well. I have searched the forum and I haven’t found anything specific about this. But I think this is something to analyze.

To whit- if Z DID NOT lick his own stamps, then the DNA would not belong to him. We do not know if this is the case. But think about it- if someone licks an envelope for someone, it would have to already be filled with a letter. Stamps you could get away with, but the flaps of the envelopes, if he did not lick them, those envelopes would have had to have already contained the letter.

So, if that is the case, there would certainly perhaps be some sort of line impression on the outer envelopes when he wrote the address. You know, if you put a card in there that was irregularly shaped, or perhaps a piece of fabric that makes the letter more puffy, for lack of a better term, you might be able to identify that by looking at the envelopes themselves, based on how the letters were folded and so on. This would probably necessitate a facsimile rendering of the letters with the same weight paper, same dimensions, same fold lines. Stuff it in there and writing on the envelope might reflect a linear impression, thereby proving that those envelopes were already stuffed and sealed before he wrote the address on the outside. Which would render the likelihood (I almost wrote "licklihood") that he did not get a third party, as it were, to lick his envelopes.
It is my recollection that some of these letters did NOT contain DNA on the envelope flaps, belonging to anyone.

So, if you imagine you are Z and you have an envelope with a piece of bloody fabric in it, are you gonna chance handing that to your mother to lick so you won’t be caught? And if you did not, then you must have used a sponge. Making the likelihood that the DNA isn’t gonna lead to anyone pertinent- being that the DNA present in the database is by and large that of criminals or missing/unidentified persons, not the public at large.