The version of sendmail that is distributed with Debian GNU/Linux 2.1 has a slight problem in the code to regenerate the aliases database. Sendmail allows any user to run sendmail with the -bi option to (re)initialize the aliases database. The user could then interrupt sendmail and leave the system with a broken aliases database.
This has been fixed in version 8.9.3-3slink1 by only allowing root and trusted users to regenerate the aliases database.
It is recommend you upgrade your sendmail package to this new version.
wget url
will fetch the file for you dpkg -i file.deb
will install the referenced file.