Has anyone been able to get Dovecot LDA working where each user has a different UID?

All the howtos on the Internet I have found set the UID of a mailbox as fixed for all users.

However I have tried to run it on the current SQL database we have (with different UID per user) and I get the following:

Jan 1 16:25:40 HOST dovecot: deliver(EMAIL): Fatal: setgid(1000(stdusers)) failed with euid=8(mail), gid=12(mail), egid=12(mail): Operation not permitted

I am calling this from Postfix with:

dovecot unix - n n - - pipe flags=DRhu user=mail:mail argv=/usr/local/libexec/dovecot/deliver -f ${sender} -d ${recipient}