For using anonymous remailers, up-to-date public key rings are a necessity.
You should fetch these at least daily and provide them to your users.
${PREFIX}/share/examples/mixmaster/mixmaster-update.pl is a sample
script facilitating this task.

All users have their own mixmaster configuration files and pools in
~/.Mix, but global defaults can be specified in
${SYSCONFDIR}/mixmaster/client.cfg. You might want to specify PUBRING and
TYPE2REL in this file to point to some sitewide files fetched, e.g. by the
above mentioned script.