Sorry for the delay in getting to this question... I've been very busy of late!
If you've installed and configured sendmail, then you are using mbox message storage, not maildirs... this may explain why you're not seeing the behavior you want. So the short answer to your question appears to be "you're barking up the wrong tree!"
Linux Journal did a write-up on the differences (QMail, Postfix, and some other mainstream mail servers can do either, but sendmail can only do mbox) back in 2007. There is a good write-up also available online at
http://www.linuxmail.info/mbox-maildir-mail-storage-formats/ (in case you are not an LJ subscriber).
Good luck! I hope this helps!
Dan
IT4SOHO