If you try to send an email message, you will most likely receive an error from your client. The error message will say something that includes this:
451 qq temporary problem (#4.3.0)
If you followed Life with qmail, you then have a memory limit set in the /var/qmail/supervise/qmail-smtpd/run file. Look for the line that contains softlimit. It should look similar to this:
exec /usr/local/bin/softlimit -m 2000000 \
This example sets the memory limit for qmail-smtpd to 2M. After all of your changes qmail-smtpd is now running the entire Perl interpreter, and ClamAV. 2M will never be enough.
Each system is different, and has different requirements. It will take some experimenting on your part to find the correct value for your system's softlimit. Do not set softlimit to some high value! You are asking for trouble if you do this. To find the minimal value for your system, I recommend the following steps:
Increase softlimit by 1M
Send a message
Repeat until you can successfully send an email
Once you have found the minimum, I recommend increasing that by 1.5M, just for times that your email server has a heavy load.
After that just create a daily cronjob that runs /var/qmail/bin/qmail-scan-queue.pl -z to cleanup any dropped SMTP sessions that may be lying around in /var/spool/qmailscan.