How to solve the ClamAV error “This version of the ClamAV engine is outdated” on Debian 5.0 (Lenny)

As of today, all ClamAV 0.94 or older installs will stop working. The Debian project has not released updates for their current stable release (5.0) yet to solve this, but there is a workaround. The Debain volatile project provides newer clamav versions. The installation steps to install ClamAV 0.95.3 on Debian lenny are:

1) Add the debian volatile repository to the sources list:

echo "deb http://volatile.debian.org/debian-volatile lenny/volatile main contrib non-free" >> /etc/apt/sources.list

2) Update the sources and install the new clamav package:

apt-get update
apt-get install clamav
apt-get -u upgrade

3) If you use amavisd-new (e.g. if you use a ISPConfig 3 server) then restart amavisd:

/etc/init.d/amavis restart