Sender gets the bounce back error message below. Recipient's email server is Exchange 2013. Sender says that they are using a servers called ATP.
Reporting-MTA: dns;com03.servicemail24.de
X-PowerMTA-VirtualMTA: whitelisted
Received-From-MTA: dns;GTLALDWVM0811 (10.6.13.136)
Arrival-Date: Wed, 12 Mar 2014 13:20:45 +0100
Final-Recipient: rfc822;recipient@recipientdomain.com
Action: failed
Status: 4.4.2 (bad connection)
Remote-MTA: dns;emailserver.recipientdomain.com (72.54.189.140)
X-PowerMTA-BounceCategory: bad-connection