Userdb by directory lookup

Hi,

I have the Dovecot (2.2.27-3+deb9u2) with LMTP and Postfix. Static userdb:

userdb { driver = static args = uid=vmail gid=vmail home=/data/vmail/user/%n allow_all_users=yes }

and passdb by LDAP, only to verify IMAP user password by bind.

Problem is, when someone sends email to non-existent@mydomain.tld, Dovecot automatically creates its home directory and Maildir.

Is there any way how deliver only when /data/vmail/user/%n directory already exists, and reject otherwise?

When I remove allow_all_users=yes, LMTP stops to deliver at all with 550 code. I probably understand why, but I cannot figure how to solve it. I cannot use LDAP to user lookup.

Milo