exim + courier + squirrelmail mail delivery in wrong directories

I just installed an E-Mailserver using exim for SMTP, courier(-imap) for fetching mails via IMAPv4 and squirrelmail as web client for the end user. Everything works fine. But when I sent an E-Mail to adress on my server, it is stored in the file /var/mail/<UNIX-USERNAME>, but not in the maildirs I created in /home/<UNIX-USERNAME>/Maildir

I am also able to log into squirrelmail. Sending mails also works. The copy of the sent E-Mail is stored in the right directoy. So, how do I tell exim to deliver incoming E-Mails to /home/<UNIX-USERNAME>/Maildir/INBOX?