What is an email server? How does it work?

What is 'Email server'?

An email server is a computer which keep email messages as an incoming server and send out messages as an outbound server. Most of large organisations have their own mail server on their office, but medium-small organisations is served by an email service provider which has good experiences and expert in computer network/server, maintaining server has quite high cost. 

The important advantage of mail server is sending message with the attachment (DOC, EXCEL, PDF File) to anyone all around the world, the recipient has no need to print out the messages and send the messages via mailbox at postoffice. It's the best way to save the world.

The senders can attach files and send to the recipients within few minutes.