Ciao @davidep, I just configured it as described in the post above (back to dovecot-lda, re-created xyz.intern, deliver locally with “accept unknown recipients” to “test@xyz.intern” for both mail domains).
This is the output of /var/log/maillog for an external E-Mail sent to an existing user (sepp.huber@xyz.de). All fine.
Feb 15 19:26:03 zeus dovecot: lda(sepp.huber@xyz.de): sieve: msgid=<e3ce5ef9-b7fc-0907-d439-56c7c75b2fe9@xxxxxx.de>: stored mail into mailbox 'INBOX'
Feb 15 19:26:03 zeus getmail: msg 1/1 (5731 bytes) msgid 0M8SlT-1cRbrd0631-00wFS7 from <info@xxxxxx.de> to <sepp.huber@xyz.de> delivered to MDA_external command dovecot-lda (), deleted
And this is the output of the same log for an external E-Mail sent to a non-existing user (user.not.existing@xyz.de). The E-Mail is not delivered. Same situation as in the opening post. 
Feb 15 19:24:02 zeus getmail: Delivery error (command dovecot-lda 3762 error (67, ))
Feb 15 19:24:02 zeus getmail: msg 1/1 (3609 bytes) msgid 0M9ZXC-1cXDXz2wUJ-00CwfH from <info@xxxxxx.de> to <user.not.existing@xyz.de>
Feb 15 19:24:02 zeus postfix/pickup[3662]: ECF23401268C3: uid=0 from=<root>
Feb 15 19:24:02 zeus postfix/cleanup[3766]: ECF23401268C3: message-id=<20170215182402.ECF23401268C3@zeus.xyz.intern>
Feb 15 19:24:03 zeus postfix/qmgr[3663]: ECF23401268C3: from=<root@zeus.xyz.intern>, size=943, nrcpt=1 (queue active)
Feb 15 19:24:03 zeus dovecot: lmtp(3771): Connect from local
Feb 15 19:24:03 zeus dovecot: lmtp(3771, root@zeus.xyz.intern): /lcdA8OcpFi7DgAA4kELwg: sieve: msgid=<20170215182402.ECF23401268C3@zeus.xyz.intern>: stored mail into mailbox 'INBOX'
Feb 15 19:24:03 zeus postfix/lmtp[3770]: ECF23401268C3: to=<root@zeus.xyz.intern>, orig_to=<root>, relay=zeus.xyz.intern[/var/run/dovecot/lmtp], delay=0.36, delays=0.24/0/0.01/0.11, dsn=2.0.0, status=sent (250 2.0.0 <root@zeus.xyz.intern> /lcdA8OcpFi7DgAA4kELwg Saved)
Feb 15 19:24:03 zeus dovecot: lmtp(3771): Disconnect from local: Successful quit
Feb 15 19:24:03 zeus postfix/qmgr[3663]: ECF23401268C3: removed
But now, additionally, all my mail sent internally to an existing user (using sendmail or dovecot client) is also sent to the account defined in the “accept unknown recipients”. 
This is the mail log:
Feb 15 20:06:18 zeus postfix/pickup[3662]: 55D18401268C3: uid=0 from=<root>
Feb 15 20:06:18 zeus postfix/cleanup[5179]: 55D18401268C3: message-id=<20170215190618.55D18401268C3@zeus.xyz.intern>
Feb 15 20:06:18 zeus postfix/qmgr[3663]: 55D18401268C3: from=<root@zeus.xyz.intern>, size=878, nrcpt=1 (queue active)
Feb 15 20:06:18 zeus dovecot: lmtp(5182): Connect from local
Feb 15 20:06:18 zeus dovecot: lmtp(5182, test@xyz.intern): 0zf1FqqmpFg+FAAA4kELwg: sieve: msgid=<20170215190618.55D18401268C3@zeus.xyz.intern>: stored mail into mailbox 'INBOX'
Feb 15 20:06:18 zeus postfix/lmtp[5181]: 55D18401268C3: to=<test@xyz.intern>, orig_to=<sepp.huber@xyz.de>, relay=zeus.xyz.intern[/var/run/dovecot/lmtp], delay=0.23, delays=0.11/0/0/0.12, dsn=2.0.0, status=sent (250 2.0.0 <test@xyz.intern> 0zf1FqqmpFg+FAAA4kELwg Saved)
Feb 15 20:06:18 zeus dovecot: lmtp(5182): Disconnect from local: Successful quit
Feb 15 20:06:18 zeus postfix/qmgr[3663]: 55D18401268C3: removed
I totally don’t know how to continue…
[edit: just garbled the external E-Mail address in the log files extract, don’t want to be a target for junk mails]