I had an issue to deal with for a client recently where messages submitted to eConnect were no longer appearing in Microsoft Dynamics GP. I did the usual things of checking the Windows Event Log and ensuring that the two eConnect services were running.
One item I tried was to submit a test message to eConnect which worked fine; so it worked for the perfectimage user I was logged in as, but no-one else. I eventually found the answer in the Properties of the econnect_incoming queue:
The permissions for the perfectimage account (Full) were still configured correctly, but, somehow, the Send Message permission on Everyone had been removed. Once this setting had been added back, people were able to submit messages to the queue without further problem.