I made some test and it looks like the creator of the
pseudo-mailing-list must be online in order messages send to this address to be received by subscribers.
Why this is needed? Why messages cannot be received if the creator of the pseudo-mailing-list is offline?
Because a pseudo-mailing-list, unlike a
chan, works almost exactly like a normal mailing-list: you send a message to the server, and then the server re-broadcasts your message to everybody.