[Dovecot] Where should I define mail_uid for deliver LDA?

[Alex]

Hello folks

I'm trying to use Dovecot deliver LDA instead Postfix virtual LDA

The last road block I have to resolve is to define mail_uid, becaus
dovecot.err log has these error messages:

May 04 14:57:19 deliver(user1 at some_domain.com): Error: User
user1 at some_domain.com is missing UID (set mail_uid)
May 04 14:57:19 deliver(user2 at some_domain.com): Error: User
user2 at some_domain.com is missing UID (set mail_uid)
May 04 14:57:25 deliver(user3 at some_domain.com): Error: User
user3 at some_domain.com is missing UID (set mail_uid)

Where exactly should I define mail_uid?
Somewhere in Dovecot dovecot.conf file?

I tried to set mail_uid in dovecot-sql.conf in user_query as follows:
user_query = SELECT maildir as mail, 51 as userdb_uid, 51 as userdb_gid,
51 as mail_uid FROM dovecot_email_maps WHERE email = '%u'

But this apparently doesn't provide that mail_uid as I expected.

Could somebody please enlighten me on this issue.

Thank you in advance,

Alex