If your mailserver stopped delivering mail and is complaining about a wrong version with the Clamav antivirus, giving an error message similar to this in /var/log/mail.log:

LibClamAV Error: cli_hex2str(): Malformed hexstring: This ClamAV version has reached End of Life! Please upgrade to version 0.95 or later. For more information see www.clamav.net/eol-clamav-094 and www.clamav.net/download (length: 169)
LibClamAV Error: Problem parsing database at line 742
LibClamAV Error: Can’t load daily.ndb: Malformed database
LibClamAV Error: cli_tgzload: Can’t load daily.ndb
LibClamAV Error: Can’t load /var/lib/clamav/daily.cld: Malformed database
ERROR: Malformed database

You can fix it in this way:

check /etc/apt/sources.list for the Debian Volatile Archive (if it is not present, add it):

deb http://volatile.debian.org/debian-volatile lenny/volatile main contrib non-free

Then run:

apt-get update
apt-get install clamav clamav-daemon clamav-freshclam