POP3 STAT command failed with RC -301

Symptom

Using Commerce:Exchange type of communication profile
Communication fail, Unable to receive data
Connect to Sterling mailbox using POP3 communication protocol on port 110.
Communication log with Net Trace Utility turned on shows QUIT command issued immediately after "Receiving:  -ERR Invalid command"
A new proxy server was just installed or the settings on an existing proxy server settings were changed
Sniffer setup from IBM Sterling Commerce:Centre network node (by IBM Sterling Network support) shows no receipt of STAT command, just a logon followed by a QUIT command.  

(NOTES:  The Net Trace Utility referred to above comes with Gentran:Director, but is not automatically installed. It is available in the Support\Utils folder of the installation files, and the file name is nettrace.exe. In order to use it, place a copy of that file on the Window Desktop, double click it, check the "Verbose" box, then click OK. It's purpose is to make the communication log more detailed.              
There exists a very similar error "POP3 PASS command failed with RC -301". The only difference in that error message is the word "PASS" instead of that word "STAT". That error has a different cause and solution. Please see article http://www.ibm.com/support/docview.wss?uid=swg21560341 Error Message
When both a send and a receive are done, the send will be done first. The start of the receive portion of the session will be indicated in the Communication Log by "Opening POP3 Session on port 110 with host...."
"POP3 STAT command failed with RC -301" shows in the Communication Log shortly after the start of the receive portion of the session.  
"Attempt to open POP3 connection failed with RC-10060" will also show in the Communication Log.
"GenMail exiting RetreiveEDIData with RC 10060" will be the last line in the Communication Log