Because deliver is delivering as uid:gid and it has to change its uid.
See http://wiki.dovecot.org/LDA (Multiple UIDs).
L??a
> -----Original Message-----
> From: dovecot-bounces at dovecot.org [mailto:dovecot-bounces at
dovecot.org] On
> Behalf Of Daniel
> Sent: Sunday, May 13, 2007 2:34 PM
> To: Dovecot Mailing List
> Subject: [Dovecot] dovecot deliver tries to setuid but why?
>
> 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