From W3C Wiki
The Semantic Inbox is an evolution of the RESTful HTTP Inbox. But what's that?
RESTful HTTP Inbox
A RESTful HTTP Inbox is an HTTP URI that accepts HTTP POST requests and brings them to the attention of the mailbox's owner.
The request must include an HTTP Content-Type header reflecting the media type of the message.
The message body itself is included in the HTTP POST Body. For example, the following is an example of posting a message to http://example.com/foo:
POST /foo HTTP/1.1 Host: example.com Content-Type: text/plain Hello world!
Additional headers which can be included: From - an RFC822-formatted sender address, and Subject a subject for the message:
POST /foo HTTP/1.1 Host: example.com Content-Type: text/plain Subject: Greetings From: email@example.com Hello world!
If the endpoint uses an HTTPS URI, then it may use an authentication system such as foaf+ssl to reliably determine the identity of the sender.
Toby Inkster has released a very basic HTTP inbox implementation - essentially just an HTTP-to-SMTP gateway.
The semantic inbox is an evolution of the HTTP inbox, presenting the same API to the message sender.
Rather than simply accepting incoming messages, it will offer to perform useful tasks with the messages if they arrive in a semantic format (e.g. RDF or Atom).
A semantic inbox could handle Pingback and read/write RDF type tasks.
Toby Inkster has begun an implementation.