[exim] routing of some users but not all

I have been running Exim 4 with popa3d for a while without any problems.
After installing Courier IMAP some of the users mail is being forwarded
correctly to their Maildir, while others is sent to the /var/mail/"user"
spool. Looking ant the log I found that for the users that were
forwarded correctly: " >
/home/"user"/Maildir/<"UID@"mydomain"."topdomain"> R=userforward
T=address_directory. But for other users I find the entry: > "UID"
<"UID"@"mydomain"."topdomain"> R=local_user T=mail_spool.
Any ideas how I resolve this?

Thanks,

Muz