When I email a userid@mail-host, one day the mail host will use on DNS
server to "resolve" the hostname-to-IP address lookup, and another day, a
different DNS.  Sometimes these DNSes are not in synch, or there is
cached info waiting to timeout so that more authoritative data from the
DNS can be used.

Similarly, sometimes the MX structure is simply obsolete and mail is
occasionally gatewayed (depending on time-of-day) to an inappropriate
host; or mail may be delivered to one of the proper hosts but its mail
configuration is FUBAR.  Finally, every part of the DNS and mail host is
fine, but the file system, authentication system, or anti-spam techniques
used by that host are WRONG (and depending upon the LISTSERV Distribute
backbone combined with the number of identical subscriber options, this
could vary).

As a co-owner of a list that distributes 30-50 mail messages per day to
3,500 subscribers on approx. 1,800 different mail addresses, I see quite
lot of bounces, some very transient.  What is most interesting is that we
have a daily digest that sometimes was not being delivered to only some
subscribers at a particular host.  The mail/system administrators were
aggressively of no help, even after I pointed them to their obsolete MX
structure.  It took nearly 4 months of whining both from their
subscribers and me to their higher-ups to get this matter resolved
(figuratively and literally).

So, with this holidaze and Y2K season upon us, I don't think you are
going to have much success in the short-term in getting this fixed.