Along with the Web, e-mail is probably the service that most people think of when they think of the Internet. Like web servers, e-mail servers have traditionally been pretty tricky to set up, and there are many guides and books on this subject. However, you will find that e-mail servers are pretty easy to set up and use under Ubuntu.
Installing an e-mail server requires you to manage the sending and receiving of e-mails. These two notions are important because they implement two different protocols—SMTP to send, and POP or IMAP to receive the messages.
When a user sends an e-mail it is in fact never in direct contact with the recipient. The e-mail goes through at least two servers that are, in order, the sending server (SMTP), called the MTA (Mail Transfer Agent), which sends it to the receiver server (POP or IMAP), called MDA (Mail Delivery Agent), possibly transiting the e-mail via other SMTP servers as well. Then, the e-mail will wait on this last server until...