lmtp userdb can't resolve users

[George Asenov]

Hello,

I use postfix with dovecot as an lmtp LDA with unix users and multiple 
domain names and mailboxes in Maildir format placed in domain directory 
like:

/home/mainuser/homes/user at domain.tld/Maildir
or
/home/mainuser/domains/domain2.tld/homes/user at domain2.tld/Maildir

which have the main user as a group and user at domain.tld/user at domain2.tld 
as owner

Postfix have
virtual_alias_maps = hash:/etc/postfix/virtual

there in virtual file there is  map like:

user at domain.tld user-domain at domain.tld

and in /etc/passwd

there are actually 2 users with the same home dir and
same UID/GID (only the username is different)


and in postfix
mailbox_transport = lmtp:unix:private/dovecot-lmtp

The issue is that when postfix passes the email for local delivery to 
dovecot lmtp it sends the username as user-domain.tld at serverhostname.tld 
but dovecot is configured with
!include auth-system.conf.ext

can't resolve this username thus fails to deliver.

I've found a workaround in the net to use custom userdb just for lmtp
like this:


protocol lmtp {
   mail_plugins = $mail_plugins sieve
     userdb {

         driver = passwd-file
         args = username_format=%n /etc/passwd
   }
}

which works but produce some warnings because there is the root user (ID 
0) and actually is a dirty workaround

Is there more elegant solution??