Because of the issue diaspora/diaspora#7422 I had a look at the specifications once again.

Traditional decentralized systems like Friendica or Diaspora, but also centralized systems like Twitter or Facebook are knowing two different message types.

The first one is the standard message. It can be addressed to the public or to a group of people or even to a single person alone. This is mostly meant for stuff that you just want to share with other people - and it doesn't really matter if all the receivers see the message at all.

The second one is more like the traditional mail. You not only get a notification about it, but also these messages are collected at a special place so that you can keep an eye on this. These messages can mostly be addressed to a group of people as well.

I haven't found any trace in the specifications on how to implement these two types - but this is crucial for replacing existing protocols.