Definition - What does Email Server mean?
An email server, or simply mail server, is an application or computer in a network whose sole purpose is to act as a virtual post office. The server stores incoming mail for distribution to local users and sends out outgoing messages. This uses a client-server application model to send and receive messages using Simple Mail Transfer Protocol (SMTP).
An email server may also be known as a mail or message transer agent.
Techopedia explains Email Server
An email server is a computer with mail transfer agent (MTA) functions. Mail is exchanged between email servers running special software, which is built around standardized protocols for handling messages and their varied (multimedia) content.
An email server receives mail from another MTA, a mail user agent (MUA) or a mail submission agent (MSA) with the details of the transmission being defined by SMTP. When an MTA receives a mail and the recipient of the mail is not hosted locally, the mail is forwarded to another MTA. Every time this happens the MTA adds a "received" trace header on the top header of the message. This shows all the MTAs that have handled the message before it arrived in the recipient’s inbox. This useful feature allows administrators to see whether an optimal path was taken.