Transport Agent PmE12transport causing Messages to back up in Submission Queue

Yesterday we found that Microsoft Exchange stopped routing any messages (incoming, outgoing, internal, external).

Upon investigation, Exchange's Queue Viewer shows that the Submission Queue had 100s of emails sitting in it and the number continued to rise as more incoming email continued to come in.

Google search suggested that the submission queue backing up is usually the result of AV/AntiSpam components.  So I started to consider PureMessage as the source of the problem.

I ran the Get-TransportAgent commandlet in the Exchange Shell which showed:

Identity                                           Enabled         Priority
--------                                           -------         --------
PmE12Transport                                     True            1
Transport Rule Agent                               True            2
Journaling Agent                                   True            3
AD RMS Prelicensing Agent                          False           4
PmE12Protocol                                      True            5

I then ran disable-transportagent PmE12Transport commandlet and restarted Exchange Transport Service and all the backed up messages pushed through and the queue dropped down to 0.  This suggests that PmE12Transport Transport Agent was causing the block.

Re-enabling the transport agent (followed by a Exchange Transport Service restart ) causes messages to back up again after about a minute.

Opening up PureMessage doesn't show up anything out of the ordinary.

I need advise as to how I can interrogate the PmE12Transport Transport Agent (sophos logs, Events etc.) to find out what it's beef is.  Any useful information will be appreciated.

Thanks,

Chris