Exchange 2013: mails in log "inboundproxy@contoso.com"

Hi,

a customer was running SBS2008, which I migrated to Exchange 2013.  The old SBS was deinstalled, demoted and removed from the domain.  This was about two months ago.

Recently I installed a new firewall system (Sophos sg) with an SMTP proxy that receives and scans and forwards all mail.  I am finding in it's queue many messages going from NO SENDER to inboundproxy@contoso.com.

The log sais this:

HealthMailboxfed50d91c7c042958d086b53575e0dd3@internaldomain.local
The entered email address could not be verified. Check the email address of the recipient and try to send your message again.  Ask helpdesk if the problem persists.


Diagnose information for administrators:

generating server: mail.internaldomain.local

HealthMailboxfed50d91c7c042958d086b53575e0dd3@internaldomain.local
Remote Server returned '550 5.1.1 RESOLVER.ADR.RecipNotFound; not found'

Ursprüngliche Nachrichtenköpfe:

Received: from mail.internaldomain.local (192.168.1.222) by mail.internalodmain.local
 (192.168.1.222) with Microsoft SMTP Server (TLS) id 15.0.1178.4; Thu, 29 Sep
 2016 09:06:12 +0200
Received: from InboundProxyProbe (127.0.0.1) by mail.internaldomain.local (127.0.0.1)
 with Microsoft SMTP Server id 15.0.1178.4 via Frontend Transport; Thu, 29 Sep
 2016 09:06:12 +0200
X-MS-Exchange-ActiveMonitoringProbeName: OnPremisesInboundProxy
X-Exchange-Probe-Drop-Message: FrontEnd-CAT-250
Subject: Inbound proxy probe
Message-ID: 
From: 
To: Undisclosed recipients:;
Return-Path: inboundproxy@contoso.com
Date: Thu, 29 Sep 2016 09:06:12 +0200
MIME-Version: 1.0
Content-Type: text/plain

Select allOpen in new window

What is happening?