> x  a "client" SMTP (typically selected by the LISTSERV(s))

We're using LSMTP on the same box.

> x  the Domain Name System (multiple DNSes, which may or may 
> not be in-sync)

I would expect bounces in this scenario, but since delivery errors are
intermittent this seems unlikely.

> x  SMTP "MX" servers

One user who complains has a very simple set-up; just one mail server to
which she has direct access and has no spam filters.

> Add onto that, the LISTSERV Distribute Backbone 

This is one area I suspected ... but in the LISTSERV logs I see "Mail posted
via SMTP to..." the problem users, so I assume they were not distributed via
other servers.

> With some anti-spam systems now configured NOT to bounce 
> "bad" e-mail, then tend to just throw it away without further 
> user or system admin involvement.

This is always my first guess. But at this point the users are viewing it as
an excuse, insisting they have no spam filters or other local mail server
problems, and that our listserv is the only source of trouble they
experience.

I enabled enhanced LSMTP logging to see if I can get any further, but I was
crossing my fingers that there is some known LISTSERV issue at there that
can magically be corrected...

--
Dave