[InXorWeTrust]

> Every message is retained forever by every BM client.

That is not true.

From the white paper [0] under section 6:

> We propose that nodes store all objects for two days and then delete them.

It's also on the FAQ[1]:

> Yes. However, if you go offline then they must come back online within 2 days of the message being sent. Nodes delete data, and do not accept data, older than 2 days.

The sender will not delete the message until it's received an acknowledge from the receiver. It instead will recompute the proof of work and resend in an exponential growing interval. This is also in section 6 of [0].

> If a node is offline for more than two days, the sending node will notice that it never received an acknowledgement and rebroadcasts the message after an additional two days. It will continue to rebroadcast the message, with exponential backoff, forever.

[0] https://bitmessage.org/bitmessage.pdf

[1] https://bitmessage.org/wiki/FAQ#Can_I_send_a_message_to_some...

[yapcguy]

The proposal in section 6 is somewhat redundant given section 5:

> ...all users receive all messages...

It would be trivial to modify the BM client to archive/save all messages it receives.