We have added two new features for virtual domain and alias users:

  1. Own-domain users can put a '*' in the alias target; this is
    replaced with the username the message was originally sent to
  2. You can set an alias target to 'user+something@domain'. Then if you
    get sent an email to 'alias+foo@domain', the final recipient will be
    'user+something.foo@domain'. That is, the two 'plus parts' are
    combined to allowing delivery to a subfolder

These advanced features are documented in the FAQ, and discussed on the forum.