keep an eye into your plesk's qmail logs
Create a file check_maillog in /usr/local/sbin or where do you prefer, make it executable and wrote in it:
tail -f /usr/local/psa/var/log/maillog /var/spool/qscan/qmail-queue.log /var/spool/qscan/quarantine.log /var/log/clamav/clamd.log /var/log/clamav/freshclam.log