postfix+dovecot and usernames different to e-mail addresses

[Benjamin]

Hi,

I want to replace an old mailserver setup with postfix+dovecot. The 
problem I'm facing now is, that the old system had usernames different 
to the e-mail address of the user. For example:

   E-Mail-Address: foo at example.com
   Username: foo-example.com

I configured postfix so that it passes mails to dovecot using the 
following command (master.cf):

   dovecot      unix   -        n      n       -       -   pipe
     flags=DRhu user=mail-data:mail-data
     argv=/usr/lib/dovecot /dovecot-lda -a $(recipient)

When dovecot now receives an e-mail from postfix it logs

   dovecot: lda(mail-data): Error: User initialization failed: Namespace
     'INBOX.': Home directory not set for user. Can't expand ~/ for mail
     root dir in: ~/Maildir
   dovecot: lda: Fatal: Invalid user settings. Refer to server log for
     more information.

My dovecot-ldap configuration looks quite simple:

   hosts = 192.168.0.1,192.168.0.2
   dn = cn=mailadmin,dc=example,dc=com
   dnpass = foo
   auth_bind = yes
   ldap_version = 3
   base = ou=users,dc=example,dc=com
   user_attrs = mailMessageStore=home
   user_filter = (&(objectClass=qmailUser)(uid=%u))
   pass_filter = (&(objectClass=qmailUser)(uid=%u))

I think dovecot does not know that the username is not the e-mail 
address, but how can I tell him?

Furthermore we have alternative addresses here, so for example there may 
be an e-mail address bar at example.com owned by foo-example.com who has 
foo at example.com as primary address.

Dovecot itself (IMAP) works fine, so here it can find the correct 
Maildir for the user. The difference is, that I connect using the 
username instead of the e-mail address in this case.

   # dovecot --version
   2.2.22 (fe789d2)
   # postconf -d | grep mail_version
   mail_version = 3.1.0
   # lsb_release -a
   No LSB modules are available.
   Distributor ID:	Ubuntu
   Description:	Ubuntu 16.04 LTS
   Release:	16.04
   Codename:	xenial

Thanks for your help
Benjamin