1. There are many reasons why the error count would go up. If you have multiple nodes running listeners for the same email account, the error count could increase if one node has a lock on the inbox and another node tried to access it at that particular moment. The logs will probably give some indication as to what happened.
2. What version of Pega is in use? This will likely require a bit of investigation.
1. I was using the multiple nodes listening to the same inbox situation merely as an example. The take away there is that there should be some indication of an error in the logs. Enabling debug on the email listener class may also be of help: com.pega.pegarules.integration.engine.internal.services.email.EmailListener
2. Enabling debug on the email listener class may help with this, but may not provide all the relevant details. If that doesn't provide much information, you could try tracing the service email rule (This would require Hfix-6340 to be installed first):
Open the service email rule (which is referenced by the email listener as the “Service Method”).
With the service email rule open, click the “play” button at the top toward the right side (next to the clipboard button).
In the drop down, click on “Trace Open Rule”. If your listener isn’t already started, start it now via SMA. Once the service email rule is called by the listener during processing, the tracer should populate with output.
I have enabled the log setting for the email listener (com.pega.pegarules.integration.engine.internal.services.email.EmailListener) and observed that email listener started polling the same message again by giving below error. Somehow system is not marking the email as seen and started processing the same email again.
Request to provide your inputs to resolve the same.
11 Mar 2016 13:33:37,442 [ EMAIL-Thread-141032] [ STANDARD] [ Inv:01.02.01] ( services.email.EmailListener) INFO EMAIL.FTCEListener.Listener|from(email@example.com)|sub(to test CE) - Messages are not processed.
11 Mar 2016 13:33:37,442 [ EMAIL-Thread-141032] [ STANDARD] [ Inv:01.02.01] ( services.email.EmailListener) DEBUG EMAIL.FTCEListener.Listener|from(firstname.lastname@example.org)|sub(to test CE) - Attempting to mark the message as 'seen'