/var/log/maillog:

Jun 17 17:55:29 fw MailScanner[12511]: Clamd::ERROR:: UNKNOWN CLAMD RETURN ./lstat() failed. ERROR :: /var/spool/MailScanner/incoming/12511
Jun 17 17:55:38 fw MailScanner[12509]: Clamd::ERROR:: UNKNOWN CLAMD RETURN ./lstat() failed. ERROR :: /var/spool/MailScanner/incoming/12509
Jun 17 17:55:41 fw MailScanner[12516]: Clamd::ERROR:: UNKNOWN CLAMD RETURN ./lstat() failed. ERROR :: /var/spool/MailScanner/incoming/12516
Jun 17 17:55:49 fw MailScanner[12505]: Clamd::ERROR:: UNKNOWN CLAMD RETURN ./lstat() failed. ERROR :: /var/spool/MailScanner/incoming/12505

Solução:

edite o /etc/MailScanner/MailScanner.conf
Faça as seguintes alterações:

De:
Incoming Work User = 
Para:
Incoming Work User = clamav

De:
Incoming Work Group =
Para:
Incoming Work Group = 0640

CentOS 4.6
Clamd 0.93.1
MailScanner 4.69.9