Dovecot LMTP rejects all users as non-existent

[Alexander Harm]

I'm running Dovecot 2.2.27 (c0f36b0) on Debian Stretch and I'm having
some trouble with LMTP delivery from Postfix to Dovecot.

The message in the logs is

Mar  9 09:21:19 mail postfix/smtpd[3295]: NOQUEUE: reject: RCPT from
some.mail.server[1.2.3.4]: 450 4.1.1 <alias at my.domain>: Recipient
address rejected: unverified address: host
my.mail.server[private/dovecot-lmtp] said: 550 5.1.1 <user at my.domain>
User doesn't exist: user at my.domain (in reply to RCPT TO command);
from=<someone at another.domain> to=<alias at my.domain> proto=ESMTP
helo=<some.mail.server>

However, testing my installation with doveadm works fine:

doveadm user -u user at my.domain
userdb: user at my.domain
  user      : user at my.domain
  uid       : 5000
  gid       : 5000
  home      : /srv/mail/vhosts/my.domain/user

doveadm auth test user at my.domain
passdb: user at my.domain auth succeeded

I'm using a static userdb and a ldap passdb:

# /etc/dovecot/conf.d/auth-ldap.conf.ext

passdb {
  driver = ldap
  args = /etc/dovecot/dovecot-ldap.conf.ext
}

userdb {
  driver = static
  args = uid=vmail gid=vmail home=/srv/mail/vhosts/%d/%n
}

And my LDAP config reads like this

# /etc/dovecot/dovecot-ldap.conf.ext

uris = ldap://127.0.0.1
dn = uid=dovecot,ou=services,dc=my,dc=domain
dnpass = *********
auth_bind = yes
auth_bind_userdn = cn=%u,ou=mail,dc=my,dc=domain
ldap_version = 3
base = ou=mail,dc=querco,dc=org
deref = never
scope = onelevel
user_attrs =
user_filter =
(&(objectclass=PostfixBookMailAccount)(mailEnabled=TRUE)(cn=%u))
pass_attrs = cn=user,userPassword=password
pass_filter =
(&(objectclass=PostfixBookMailAccount)(mailEnabled=TRUE)(cn=%u))
iterate_attrs = cn=user
iterate_filter = (&(objectclass=PostfixBookMailAccount)(mailEnabled=TRUE))

Sending the mail straight to user at my.domain works. Sending it to another
alias works as well. Is there some caching problem and if so how to
delete the cache (reboot didn't work)?

Any pointers are highly appreciated.