404 for some mbox archives

Hi,

I'm getting 404 when attempting to download some of the mbox archives.

For example

  http://www.postgresql.org/list/pgsql-admin/mbox/pgsql-admin.199805

works just fine, but this

  http://www.postgresql.org/list/pgsql-admin/mbox/pgsql-admin.199804

fails with 404. Attached is a list of more such examples.

BTW how does this (mbox archives behind authentication) protect us
against spam?

regards
Tomas