Symptom

In this case, the AS2SendAndProcessSyncMDN Business Process fails on BP Step 'HTTP Client POST Service'.
In the clients environment, the Perimeter Server is remote, and resides in a DMZ.

The PerimeterServer was confirmed to be the same build version as the GIS, using the required JDK.
This was a new GIS implementation {New install, first Trading Partner, and no previous AS2 traffic}.

--- from the BP Status Report:

Name: AS2SendAndProcessSyncMDN     Instance ID:123456     Service Name: HTTP Client POST Service    
  HTTPClientPostService resumed with error.
HTTPClientAdapter [HTTPClientAdapter]
ServiceStartTime [2008-MM-DD HH:MM:SS.SSS]
ServiceEndTime [2008-MM-DD HH:MM:SS.SSS]
SessionToken [HTTPClientAdapter_HTTPClientAdapter_node1_123456789012345:67

HTTP Status Code: -1
HTTP Reason Phrase: Internal Error: IOException Encountered

--- from the httpclient log:

[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE CustomConnectAgent.connectFailed() - Received signal from Perimeter due to fail to make connection with message [Could not complete connection to specified host]
[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE CustomConnectAgent.connectFailed() - encountered error due to [Could not complete connection to specified host]
[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE CustomConnectAgent.connectFailed() - Received signal from Perimeter due to fail to make connection with message [Could not complete connection to specified host]
[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE CustomConnectAgent.connectFailed() - encountered error due to [Could not complete connection to specified host]
[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE CustomConnectAgent.connectFailed() - Received signal from Perimeter due to fail to make connection with message [IOException Encountered]
[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE CustomConnectAgent.connectFailed() - encountered error due to [IOException Encountered]
[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE CustomConnectAgent.connectFailed() - Received signal from Perimeter due to fail to make connection with message [IOException Encountered]
[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE CustomConnectAgent.connectFailed() - encountered error due to [IOException Encountered]

--- from the Perimeter.log:

[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE Connection connect[0.0.0.0/0.0.0.0:0=>/123.45.67.89:5555] closed due to CloseCode.COULD_NOT_CONNECT
[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE A ByteBuffer was garbage collected.
[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE A ByteBuffer was garbage collected.
[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE Connection connect[0.0.0.0/0.0.0.0:0=>/123.45.67.89:5555] closed due to CloseCode.COULD_NOT_CONNECT
[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE A ByteBuffer was garbage collected.
[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE Connection connect[0.0.0.0/0.0.0.0:0=>/123.45.67.89:5555] closed due to CloseCode.COULD_NOT_CONNECT
[YYYY-MM-DD HH:MM:SS.SSS] ERROR 000000000000 GLOBAL_SCOPE A ByteBuffer was garbage collected.

--- from the Pslogger.log(from remote DMZ PS):

[YYYY-MM-DD HH:MM:SS,SSS] ALWAYS PhysicalConnectionManager connections: active: 0, listening: 1, loopback: 0
[YYYY-MM-DD HH:MM:SS,SSS] ALWAYS PhysicalConnectionManager connections: active: 0, listening: 1, loopback: 0
[YYYY-MM-DD HH:MM:SS,SSS] ALWAYS PhysicalConnectionManager connections: active: 0, listening: 1, loopback: 0
[YYYY-MM-DD HH:MM:SS,SSS] ALWAYS PhysicalConnectionManager connections: active: 0, listening: 1, loopback: 0
[YYYY-MM-DD HH:MM:SS,SSS] ALWAYS PhysicalConnectionManager connections: active: 0, listening: 1, loopback: 0
[YYYY-MM-DD HH:MM:SS,SSS] ALWAYS PhysicalConnectionManager connections: active: 0, listening: 1, loopback: 0