vpopmail and procmail
I tend to get a bunch of questions on getting these two to work together. It's not quite trivial, but indeed possible...
This is the vpopmail and procmail FAQ, not the qmail and procmail FAQ. It only covers what's specific for vpopmail.
However, google couldn't find any good qmail + procmail pages, so one beautiful day I might just... Until then, at least sound prior knowledge of qmail and procmail is required for this page.
Setting up the files
The main problem is that this will not work with vdeliver. This means that you will have to create a .qmail file for every one of your users - something that will look funny in vqadmin, if you use that.
Imagine you have a virtual domain, say "almqvist.net". "johan" is a user in that domain. Go to the domain directory and create a file named ".qmail-johan". Into that file, put the following:
| preline procmail -t ./johan/Maildir/procmailrc
Next, set up the users procmailrc (in the example, "/var/qmail/domains/almqvist.net/johan/Maildir/procmailrc") and you're ready to go. Note that all pathnames are relative to the domain directory, not the user's directory.
I hope this works... send me mail if something seems wrong...
Warning: Many people advocate using safecat to do the delivery as procmail is not 100% Maildir compliant. I have never had a problem, though...
Be sure to read this discussion also, especially if you use quotas or other vpopmail features.