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.