Yesterday I was updating a Ubuntu 8.10 Intrepid server version, and I discovered it is not more supported so the source list “points” to URLs no more valid to get packages and package list. In particular I was installing exim4.
The source list entries were pointing to archive.ubuntu.com and securety.ubuntu.com (source list is /etc/apt/sources.list).
To solve the problem I changed all the entries to old-releases.ubuntu.com and all “not found” errors went away (phew). The server is a production server and I cannot do a dist upgrade (at least not to install only exim4).