I suspect, given how I had to OK your message to the list, that your
mail server is setting "Sender= [log in to unmask]" (or "root@wherever")
on outgoing mail.  This causes your backbone updates to go out as
if they were coming from ROOT rather than from LISTSERV, which causes
the X-TBREG operation to fail.  Bottom line is that this is broken
behavior, at least as far as LISTSERV is concerned, and a quick read
of RFC822 would seem to back this up.  ("root" is not acting as an
agent for "listserv", so it should not be the "Sender:".)

Nathan