Normally your ISP chooses and maintains the email server software and runs it on his computer. It nothing you have to think about other that which protocols its uses, normally SMTP to send mail and POP3 to receive. Sometimes you see IMAP for receiving or POP3 used for sending as well. Usually you have some sort of authentication — usually a userid and password to keep other out of your private email or from relaying spam.

You can run your own mailserver on your own machine. This lets you pick up and send mail much faster. Further, you are in charge. You don’t have to live with your ISP’s quotas or other rules. However, you need some technical expertise to set it up and keep it running 24 hours a day to receive mail. You also are then free to incorporate anti-spam software into your mailserver. James from Apache is a free, pure-Java mailserver.