Zumbox and ECM are very different.
With ECM, you purchase a mailbox and get a real postal address in many destinations (USA and others). With every letter that is sent to your address - you get an email notification with a scan of the envelope. You can instruct ECM open and scan the content of your letter or you can instruct them to recycle it instead. It's a very convenient service because it saves you the need to handle paper mail.
Zumbox offer a new marketing approach. They try to get people to sign up and try to get marketers to send mail to those who registered. I guess their ultimate goal is to render the USPS worthless.