Not sure why they wouldn't use XMPP for this?
Also, there's another app being worked on called Stamp is a similar messaging system as well, but uses an HTTP based transport instead of SMTP.
The things that set Stamp apart is that is has:
* Spam mitigation in the base layer so relay operators don't need to deal with it * An indirect address lookup system so you don't get tied to one Stamp provider. * No attachment to PII to start accounts. (No phone number or email required)
Check out the whitepaper, it's not very long: