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, and when a message can not be delivered, that MTA must send a bounce message back to the sender, indicating the problem.

Answer this question based on the passage: all email services are based on which software model? Let's have some stream of consciousness first.
The relevant information is: 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). So, the answer is the Simple Mail Transfer.