[Dovecot] Permission denied

Hello all,

I'm a new user of dovecot and I've got a permission denied problem. I've read the VarMailDotLock wiki paper but there is still a security problem. If I enter : chmod a+rwxt /var/spool/mail it works but I think it's very dangerous to have thaht rights. So I've tried to edit the dovecot.conf file and to change the mail_extra_groups = mail and to change the rights : chmod +t /var/spool/mail but it doesn't work !

The mails are in /var/spool/mail/unix_user_name

Could you please help me to configure the /var/spool/mail to have a good security and to allow people to receive their mails ?

Thanks

Frederic