which protocol is used to send email messages from a mail client to a mail server
Simple Mail Transfer Protocol Email is submitted by a mail client (mail user agent, MUA) to a mail server (mail submission agent, MSA) using SMTP on TCP port 587. Most mailbox providers still allow submission on traditional port 25. The MSA delivers the mail to its mail transfer agent (mail transfer agent, MTA). Often, these two agents are instances of the same software launched with different options on the same machine. Local processing can be done either on a single machine, or split among multiple machines; mail agent processes on one machine can share files, but if processing is on multiple machines, they transfer messages between each other using SMTP, where each machine is configured to use the next machine as a smart host. Each process is an MTA (an SMTP server) in its own right.
Address Resolution Protocol The Address Resolution Protocol is a request and response protocol whose messages are encapsulated by a link layer protocol. It is communicated within the boundaries of a single network, never routed across internetwork nodes. This property places ARP into the link layer of the Internet Protocol Suite.[2] Although ARP was not developed in the OSI model, it is often described here as residing in layer 3, being encapsulated by Layer 2 protocols.[citation needed]