all email services are based on which software model
Email Messages are exchanged between hosts using the Simple Mail Transfer Protocol with software programs called mail transfer agents (MTAs); and delivered to a mail store by programs called mail delivery agents (MDAs, also sometimes called local delivery agents, LDAs). Accepting a message obliges an MTA to deliver it,[56] and when a message cannot be delivered, that MTA must send a bounce message back to the sender, indicating the problem.