As far as I understood, the transactions must be build interactively between sender and receiver. This would mean that you could only send grin to people how are online. Is this correct? And if so, is this going to change?
What this means is that both parties must participate in a transaction. With Bitcoin, and hence with most blockchains, you can send to an address; that address can be a cold wallet, or could not exist at all, in which case your cybercoins are gone.
With MimbleWimble both parties must participate (interact) to form a transaction, but this can happen asynchronously.
In other words, both parties must be online, but not necessarily at the same time.
> you can send to an address; that address can be a cold wallet, or could not exist at all
I don't think this is accurate since the other party must participate.
The difference between 'both parties must be online at the same time' (true of, say, a phone call) and 'both parties must participate, even if it's during different weeks' is a substantive difference, I only wished to indicate that MW requires the latter, not the former.
No. interactive != online. You can exchange information over any comms method, i.e. email, carrier pigeon, message in a bottle, or... Grinbox: A a federated transaction relay specifically designed to facilitate grin transaction building. It supports asynchronous message delivery. (Disclosure: I'm part of the team building it.)
Best way to try it out in action is via https://github.com/vault713/wallet713
An update on what else we're up to https://medium.com/vault713/hello-from-vault713-6f7b01d9abec