http://www.qmail.org/man/man8/qmail-remote.html
Using a control file "smtproutes" it can be controlled which server to be sent for email of which domain without consulting DNS - so make internal route control easier and smarter.
Symptom: get rebound email like this:
Hi. This is the qmail-send program at xxx.xxx.xxx. I'm afraid I wasn't able to deliver your message to the following addresses. This is a permanent error; I've given up. Sorry it didn't work out. <xxx.xxx@xxxx.xxxx.xxx>: CNAME lookup failed temporarily. (#4.4.3) I'm not going to try again; this message has been in the queue too long.
Solution:
Install Oversize DNS Patch by Christopher K. Davis
References:
https://www.antagonism.org/mail/simscan.shtml
http://qmail.jms1.net/simscan/
First step to investigate a problem: adjust logging level
export SIMSCAN_DEBUG=2
Above reference aims to solve a problem I entered - clamav change of virus database; it's pretty long time ago and simscan did not has a new release, while the reference providing patches; it seems simscan is not well / actively maintained;
November 29, 2012 - I encounter a problem seems to be that, while there is attachment in the email which name has non-ascii character, strange things happen and simscan found illegal attachment that should not exist.