After installing simscan I was no longer able to send any message. The following message was being spitted out upon sending an e-mail through this SMTP-server:

Looking at the log files, I saw the following line appearing on each connect in /var/log/qmail/smtpd/current:

Not really clear, but it turned out that clamd was not running, something simscan heavily depends on. Make sure that clamd is running and if not, try to track down why it is not.