[Dovecot] dovecot deliver tries to setuid but why?

[Daniel]

Hi!

I've set up postfix(2.4.1) + dovecot(1.0-cvs) + dovecot-lda(1.0-cvs).

Dovecot's deliver is running as vmail:vmail (according to postfix's 
master.cf).

Now the problem (when receiving mail):
deliver(leva): auth input: leva
deliver(leva): auth input: uid=8006
deliver(leva): auth input: gid=8000
deliver(leva): auth input: home=/var/mail/virtual/leva
deliver(leva): setuid(8006) failed: Operation not permitted

Every virtual user has different uids, but the gid is the *same* for 
everyone. The gid is 8000(vmail) for everybody, and deliver runs as 
8000:8000 (vmail:vmail). vmail group has write access to the users' 
maildirs. So why does deliver try to setuid when it has group write 
permission to the destination directory?

Thanks!

Daniel