Source: http://forum.swsoft.com/printthread.php?threadid=40173

If qmail-smtpd is spawning many processes and comsumming all processing capacity on a server check the name of the *.pem files in /var/qmail/control. If the files are not:

 dh512.pem
 dh1024.pem

Rename them (usually I've seen dhparam512.pem and dhparam1024.pem) accordingly, or, symlink them:

 # cd /var/qmail/control
 # ln -s dhparam512.pem dh512.pem
 # ln -s dhparam1024.pem dh1024.pem
 # service xinetd restart
 # killall qmail-smtpd  # <-- optional, but I do this to clear the orphans

From the SW-Soft forums, the explanation given for this event is:

 These 2 certificate files are used by qmail-smtpd for TLS sessions. 
 qmail tries to open them on each incoming connection, 
 so if they are not available qmail has to generate them on-the-fly which occupies all cpu time.

I saw this twice last night and this information resolved the problem both times.