This is SharePoint 2013 in built bug. We need to apply this solution after load balancing. We observed load balancing of multiple web fronts environment for 2007, 2010 and 2013. In MOSS 2007 and 2010, Incoming email service is perfectly load balanced. When more than one front-end server is set up to process incoming email messages on a MOSS 2007 and SharePoint 2010 farm, all of the WFE servers can process email messages and can fail over to any other server. Where as in SharePoint 2013 environment, we use shared/network drop folder when incoming email service runs in multiple WFE.