Edge Transport Server Troubleshooting

I have an Exchange 2010 organization.  It was upgraded from Exchange 2003 (coexistent mode).  For the last few months everything has been working really well.  3 days ago, the Edge Transport server just stopped working.  The PC itself is fine, but mail flow has completely stopped.  After trying to diagnose the problem via the Exchange Toolbox Mail Flow Troubleshooter I am not given any leads to what might be causing the problem.  The final result from the troubleshooter is a 5.3.3 Unrecognized command error about not being able to send mail.  Not much help...  Desperately needing email flow again, I removed the edge server and have been using the Hub Transport only.  The email flow has returned, but riddled with spam again.

Noteworthy points...
No emails were building up in the Edge Server queue.
Nothing was changed up to or during the system failure.
The Edge Transport server is a Windows 2008 R2 PC located in the DMZ.
The system was working very well before this issue arose.
Recreating the edge subscription did not fix the issue.
All appropriate ports are open and correct firewall policies are in place.
Unbelievably, I've has some cyber-attacks from a disgruntled former contactor we've stopped using.. Possibly even on this machine??


I've read info about this issue happening to others and how there systems were running out of disk space.  As far as I can tell my Edge Server has over 50gig of hard drive space left.  

What would be some troubleshooting steps anyone could recommend to help solve this problem?  The best practices analyzer and mail flow troubleshooter do not lead me to a solution.