A Mail Server (usually) is a dedicated mail delivery machine that serves other mail server machine by receiving and storing emails for transmission to other computers.

It is independent of your PCs (unless you are using you PCs as a mail server) and sit somewhere in a building--managed by an Internet Service Provider or corporate IT department.

You can use any PCs to get at the Mail Server to send or receive emails from any location--if you can access the mail server SMTP or use your own computer as a mail server.
See: PCs Mail Server

Email servers uses a variety of protocols to communicate with the Internet, however the most common protocols are SMTP, POP3 and IMAP4

SMTP - Simple Mail Transport Protocol a protocol governing email transmission and reception (sending and receiving emails between mail servers)

POP3 - Post Office Protocol Version 3 rfc 1939 Used for delivering messages to mail readers. See SMTP POP3 Error return codes or download rfc 1939.txt file.

IMAP4 - Internet Message Access Protocol Version allows a client to access and manipulate email messages on a mail server and permits manipulation of remote message mailbox and delivering messages to mail readers--or relay the email messages to SMTP mail servers using POP3