Mail transmission in the Internet did not arise as a result of well-planned engineering processes; rather, it grew and evolved in a more organic way.
At present, most mail submission and delivery throughout the Internet is done by means of the Simple Mail Transfer Protocol, or SMTP. SMTP was originally defined as a message transfer protocol - that is, a means to route (if necessary) and deliver mail by putting finished (i.e. complete) messages in a mailbox. Originally, users connected to servers from terminals, and all processing occurred on the server. Today, a split-MUA (Mail User Agent) model is common, in which MUA functionality occurs both on the user's own system, and the server.
In the split-MUA model, the process of getting a message to the user is accomplished by access to a mailbox on the server, using protocols such as POP and IMAP. Also, in the split-MUA model, the user's access to his/her message is based on a "Message Pull" paradigm, in which the user is required to explicitly poll his/her mailbox to retrieve mail. Message delivery based on a "Message Push" paradigm, in which mail is delivered directly to the user without polling, is presently not supported.
Despite its original definition as a message transfer protocol, in the split-MUA model, SMTP is often used for message submission. The widespread use of SMTP for submission has become a reality, regardless of whether this is a good or a bad thing.