I've followed the following HOWTO:
Virtual Users And Domains With Postfix, Courier And MySQL (+ SMTP-AUTH, Quota, SpamAssassin, ClamAV)
but after setting everyting up I get the following error:
debian:/var/log# telnet localhost 110
Connected to localhost.localdomain.
Escape character is '^]'.
+OK Hello there.
+OK Password required.
-ERR Maildir: No such file or directory
Connection closed by foreign host.