If you use a mailserver wil very low ram (< 500 MB) and a slow CPU then it might be nescessary to disable the spam- and antivirus filter functions in ISPConfig 3 as amavisd and clamav might be too heavy
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
ISPConfig 2 uses ClamAV (http://www.clamav.net) as virus scanner and the E-mail Sanitizer (http://www.impsec.org/email-tools/procmail-security.html) as content filter.