Issue
After upgrading my SharePoint 2010 databases to SharePoint, I noticed that incoming email was not processing. The timer job was running every minute as it should, but it would throw and error for each message in the drop folder:
Message: Error occurred processing 1 message(s).
In my case, my 2013 farm was running the December 2014 CU, which is version 15.0.4675.1000.
Cause
There is a bug that was likely introduced in the December 2014 CU that incorrectly checks the Sandbox Solution Code Limit on the site’s quota. The key here is that the quota template needs to have values specified in the Sandbox Solutions with Code Limit section.
This won’t work
This will work:
Resolution
Update the site’s quota template to include both storage and sandbox solution code limits.
A bug report has already been filed with Microsoft on this issue.
UPDATE 4/15/15: A fix for this issue is included in the April 2015 CU for SharePoint 2013
Dan Myhre's Blog 