Did your Qmail server failed to receive a message including an attachment and showing this output in the SMTP logs?
policy_check: remote sender@domain.tld -> local recipient@domain.tld (UNAUTHENTICATED SENDER)
policy_check: policy allows transmission
qmail-smtpd: qq soft reject (mail server temporarily rejected message (#4.3.0)): MAILFROM:<sender@domain.tld> RCPTTO:recipient@domain.tld
tcpserver: end 0000 status 000
In this particular case, the version of ClamAV engine is involved. A bug in version 0.97.5-1.4.1 end up rejecting the file scan under high load.
To resolve this issue, update to version 0.98.1-1.4.6 or greater.