I have two lists, each with 20,000 names on them. The algorithm Mailman
uses, to created the list e-mail, is inefficient.
There was a program that was used in conjunction with Majordomo called
Bulk-mailer which did a tremendous job.
In effect, it bundled addresses based on domain name, and let you limit how
many domain names were sent in the same e-mail. This way, on a 20K list,
maybe 500 e-mails would go out, but they would do it in a fraction of the
time Mailman currently does it (it takes about 6 - 10 hours).