BM-2D9tpN1g82FC9hhC8eof5BiH3Dc2NCTzQ6
Just tested it by sending a message to someone from Reddit. They must not be on though, no reply. My understanding is that eventually, if he does not reply the message vanishes?
If he doesn't come ONLINE, the message disappears. They can only be downloaded dependably for two days after they are sent.
The message is only kept in the network for around 2 day, but if you don't get an ACK message after that your client will resend the message. Broadcast messages are gone after 2 days, but direct messages will be resent after that 2 days as long as the sending client is online.