yesterday I "inherited" an ancient debian 3.1 machine to which I am supposed to add a few functionalities (mail, among them)
is there a safe way to upgrade it to a superior version ? if not, is there a repository for its debs ?
Sarge is no longer receiving security updates. There are security issues that will not have been patched on sarge. If the machine is available on the Internet, you should strongly consider to upgrading to at least etch, and preferably lenny. If it isn't obvious make a backup first.
You should be able to upgrade, but not directly. You'll need to go from sarge->etch->lenny. Since it is remote you'll want to make sure they hardware will be supported in the newer OS.
You can use this in your /etc/apt/sources.list if you really want to continue to use sarge for right now.
There are some "archive" servers. You could of course dist-upgrade it to etch, then dist-upgrade etch to lenny, but it's a bit of a stretch. You'd better make a list of installed packages with
saving /etc and other data files, then install a new lenny and reinstall additional packages with