SMTP - Mail Protocol

Used to exchange mail messages Between (sending) clients and mail servers, and between mail servers.

Works as a conversation between the client and the server.

Separates the address part from the letter itself - which allows supporting BCC and similar fields.

Allows a client to send a message via a local mail server, instead of directly to the target address. This is important to support dial-up clients, which are not connected to the Internet most of the time.