In Mimblewimble there are no addresses, and transactions need to be built interactively by the participating parties. This requires that both parties set direct socket connection for every transaction, which can be impractical, and doesn’t allow for transactions while the receiving party is offline.
A Secure Bulletin Board (SBBS) system solves this problem, enabling offline transactions, which makes the user experience the same as other cryptocurrencies such as Bitcoin. With SBBS, client wallets can exchange messages by store-and-forward nodes.
With SBBS, messages are private when addressed to an individual, such messages are encrypted with a public key before they are sent, without having to trust relaying nodes. Other messages can be set to public and be unencrypted, or sent to specific channels.