Any computer can access a SMTP Mail Server
to send or receive emails from any location
and you can use any PCs to get at the Mail Server.

Can I use my (desktop) PCs as an email server
and bypass my ISP?

Using your PCs as a mail server is a simple process. All you need is an Internet access account and a mail server program--a freeware version can be downloaded from www.argrosoft.com See: PCs mail server

The function of a mail server program is to receive or sent emails from your PCs directly to your recipient's mailbox residing their ISPs mail server without having to go through (bypass) your ISPs mail server.

Mailbox - a character string which identifies a user to whom the mail is to be sent and normally consists of user specifications and host name, ie: me@mymailserver.com

What are mail (email) servers?

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.

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