set up a virtual email-domain with linuxconf. every now and then (say twice a week) the vpop3d just stops saying: inetd[426]: pop-3/tcp server failing (looping), service terminated.
This is not a bug in vpop3d. This is a normal message from inetd. inetd is the one starting vpop3d for every session. It tries to avoid a denyal of service attack. It limits the number of connection to 40 per minutes. If you get more, it is assuming that you are either under attack or that the vpop3d service is failing so the client are retrying and retrying. So it disable the service for 5 minutes. 40 connections per minutes is generally too low for many servers, especially ISPs. In the file /etc/inetd.conf, locate the vpop3d line and change the nowait word for nowait.100. Here 100 is just a number bigger than 40. I assume that if you get this problem twice a week, you are just border line with 40 (the default) so 100 will be plenty enough.