content-identifiers

Re Content-IDentifiers:

Please check out RFC 2111, which defines URL schemes for the
values contained in message Content-ID and Message-IDs. It was
intended for use in email, but could be used in any kind of
'push' situation.

>    Both message-id and content-id are required to be globally unique.
>    That is, no two different messages will ever have the same Message-ID
>    addr-spec; no different body parts will ever have the same Content-ID
>    addr-spec.  A common technique used by many message systems is to use
>    a time and date stamp along with the local host's domain name, e.g.,
>    950124.162336@XIson.com.