I wonder if there is mechanism for vitual mail box(implemented by
vpopmail) which act as .qmail file that can trigger scripts by mail
delivery? Do I need to create actual linux account for each virtual
user for this propose?
Sure, check out the configure options, one of them enables maildrop,
which essentially means it will run a maildrop mailfilter on incoming
mail. You can put your logic in there based on whatever you want. THis
is an alternative to manually doing what Christian says.