@strypey :]

> Can you retract a private mail message? How?
No. Not in any openly federated system because of the laws of physics (you know, time only goes forward etc.).

> Can you retract a post? How?
No. Not in any openly federated system because of the laws of physics (you know, time only goes forward etc.).

The answers are also true for any other, also closed and centralised, system eventually being sent (copied) to any machine the message author does not control.