Earth Class Mail. As we are renting our house, we needed to have a PO Box to forward all our US letters. Earth Class Mail provides you with a PO Box number, and we can check whatever the letters/packages sent to the PO Box address online. Once you receive a letter or package, you get an email saying a new item has been arrived. You go online and check the envelop and if you wish, you can request a scan (either in b&w or in color). They can also send you the physical letters to wherever you are. And, that's how I got my letters today!
This service is a great one to have, especially if you have to be outside of the US for a long time.