AA.com needs to make a deposit ETH/ETC to contract of host BB.net for open a "trusted channel". Each channel has a size = the number of allowed emails per hour, eg 100 emails/hr. Size depends from a deposit amount.
Host BB.net can lock this channel and it's deposit.
Trusted channel have infitity life cycle before it will be closed or locked. It means you can open channel for 200 emails/hr and send there within a years with such speed, but if you start to send not an "important" emails - deposit will be locked.
Admin of BB.net can unlock and return deposit to AA.com manually, in case if there was some mistake. In other case deposit goes to cover transaction fees of "positive closed channels" or to the ethereum developers. In other words - locked deposit will be burnt or send as a donation to someone (who is not an interested party at all).
AA.com (sender) pays additional amount = transaction fee to close a channel. When the recipient (BB.net) blocks a channel and spends a fee for such transaction - the same reserved amount is returned to its public key at the same call. This makes the process of locking channel free for host BB.net
One host have one contract address for all incomming emails(hosts).