SBS 2011 - Emails appearing to send but return with Delivery has failed to these recipients or groups message

Been made aware today of the following problem on a small business server 2011. It started on Friday - messages appearing
to send but coming back with the following message.

Delivery has failed to these recipients or groups:
Paul  (Paul@recipientdomain.co.uk)
The server has tried to deliver this message, without success, and has stopped trying. Please try sending this message again. If the problem continues, contact your helpdesk.
Mark  (Mark@recipientdomain.co.uk)
The server has tried to deliver this message, without success, and has stopped trying. Please try sending this message again. If the problem continues, contact your helpdesk.
Warwick and Lynn (recipientaddress@gmail.com) (recipientaddress@gmail.com)
The server has tried to deliver this message, without success, and has stopped trying. Please try sending this message again. If the problem continues, contact your helpdesk.





Diagnostic information for administrators:
Generating server: SERVER.sbs.local
Paul@recipientaddress.co.uk
#550 4.4.7 QUEUE.Expired; message expired ##
Mark@recipientaddress.co.uk
#550 4.4.7 QUEUE.Expired; message expired ##
recipientaddress@gmail.com
#550 4.4.7 QUEUE.Expired; message expired ##
Original message headers:
Received: from SERVER.sbs.local ([fe80::649a:3518:b3bc:e1c]) by
 SERVER.sbs.local ([fe80::649a:3518:b3bc:e1c%11]) with mapi id
 14.01.0438.000; Fri, 11 Dec 2015 16:16:30 +0000
From: User <dan@mycompanydomain.uk.com>
To: Paul  <Paul@recipientaddress.co.uk>, Mark
      <Mark@recipientaddress.co.uk>
CC: "Warwick and Lynn (recipientaddress@gmail.com)"
      <recipientaddress@gmail.com>
Subject: RE: Deliveries due next week (W/C 14th December)
Thread-Topic: Deliveries due next week (W/C 14th December)
Thread-Index: AdEyfvzWWwBUDlKIT8udyNTjsoIIJwAAAVpAAGuz2AAAAFRxcA==
Disposition-Notification-To: Usernam <dan@mycompanydomain.uk.com>
Date: Fri, 11 Dec 2015 16:16:29 +0000
Message-ID: <0AAEEA9084EA11499051F8014B67EFBA404C6289@SERVER.sbs.local>
References: <0AAEEA9084EA11499051F8014B67EFBA404C51E4@SERVER.sbs.local>
 <DB5PR03MB13687047D19A2C31FD29D1F09EEA0@DB5PR03MB1368.eurprd03.prod.outlook.com>
In-Reply-To: <DB5PR03MB13687047D19A2C31FD29D1F09EEA0@DB5PR03MB1368.eurprd03.prod.outlook.com>
Accept-Language: en-GB, en-US
Content-Language: en-US
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
x-originating-ip: [192.168.16.11]
Content-Type: multipart/related;
      boundary="_004_0AAEEA9084EA11499051F8014B67EFBA404C6289DESMAT1sbslocal_";
      type="multipart/alternative"
MIME-Version: 1.0

Nothing has changed. The sbs self certificate expires on the 16/12/15 so I ran the fix my network wizard yesterday just selected that
to fix it. This was before I knew abou the problems.


Really need some help on this one, thanks in advance.