Mail system message that raised the alarm:
Reporting-MTA: dns; mail01.xxxx.xx
X-Postfix-Sender: rfc822; xxxxx at xxx .xy
Arrival-Date: Wed, 13 Apr 2016 16:30:34 +0200 (CEST)
Final-Recipient: rfc822; xxxxx @ xxx .xy
Original-Recipient: rfc822; xxxxxx at xxx .xy
Remote-MTA: dns; mail02.xxxx.xx
Diagnostic-Code: smtp; 550 5.1.1 <xxxxxxx at xxx .xy >: Recipient address
rejected: User unknown in local recipient table
From my point of view indexing, even if not needed, may never do the
fail, isn't it?
But seems something weird happened.