Email Alerts from PI server with error INBOUND_DELIVERY_ERROR

Hello PI gurus

In the past week I had recieved thousands of alerts (2 different times) and the error that I saw was INBOUND_DELIVERY_ERROR.

However, when I logged into the system and checked SXMB_MONI, all the messages were successfully processed indicating some temprorary glitch (either network related or server related). However, when I tried to search for this error on internet, I couldn't find any useful information.

Does anyone know the exact meaning of this error and why this might have been caused. Any comments/feedback will be highly apprecaited.

Thanks
Ram