Inbound Proxy Probe Undeliverable (Health Mailbox)

Hi, we've recently switched our anti spam filter to Mimecast and I've started noticing in the 'Bounce' section an email every couple of minutes being caught as undeliverable, shown below. This was most likely happening before, but our previous anti spam filter was useless!

I'm aware of the HealthMailboxes and from my understanding they should be receiving these emails, rather than attempting to send them externally. I've managed to catch one of these emails and it contains the following content:

Delivery has failed to these recipients or groups:
HealthMailbox3ac30e4d99434adaa06f83ce3062fb1f@OurDomain.co.uk
The email address you entered couldn't be found. Please check the recipient's email address and try to resend the message. If the problem continues, please contact your helpdesk.


Diagnostic information for administrators:
Generating server: mailserver2.OurDomain.lan
HealthMailbox3ac30e4d99434adaa06f83ce3062fb1f@OurDomain.co.uk
Remote Server returned '550 5.1.1 RESOLVER.ADR.RecipNotFound; not found'
Original message headers:
Received: from mailserver2.OurDomain.lan (192.168.0.10) by
 mailserver2.OurDomain.lan (192.168.0.10) with Microsoft SMTP Server (TLS) id
 15.0.1130.7; Mon, 25 Jan 2016 11:15:44 +0000
Received: from InboundProxyProbe (127.0.0.1) by mailserver2.OurDomain.lan
 (127.0.0.1) with Microsoft SMTP Server id 15.0.1130.7 via Frontend Transport;
 Mon, 25 Jan 2016 11:15:44 +0000
X-MS-Exchange-ActiveMonitoringProbeName: OnPremisesInboundProxy_2
X-Exchange-Probe-Drop-Message: FrontEnd-CAT-250
Subject: Inbound proxy probe
Message-ID: <1125c87ee52942dea2fd46d60221eec2@mailserver2.OurDomain.lan>
From: <inboundproxy@contoso.com>
To: Undisclosed recipients:;
Return-Path: inboundproxy@contoso.com
Date: Mon, 25 Jan 2016 11:15:44 +0000
MIME-Version: 1.0
Content-Type: text/plain

The listed mailbox does exist and contains the SMTP address. I've followed steps to rebuild the mailboxes and they are recreated as expected, but then within moments of them all being recreated the issue starts happening again.

Our system contains 3 mailbox servers all members of a DAG, each containing a copy of 4 databases. They are all running Exchange 2013 CU10.

Thanks for any help.