i use qmail in combination with vpopmail from inter7 on my servers.

the most start scripts for qmail use tcpserver and tcpserver uses recursive dns for every request.
so if there is a lot of traffic or spam or whatever, tcpserver sometime takes to long to resolv these ip addresses and no more connections are able to be opened in that time.
the resultion is to set the params like this:

tcpserver -H

the -H means don’t resolve the ip.