The idea of exchanging mail messages with another mail system sounds simple enough but getting it to work can be a nightmare. As mentioned, the standards used by a particular mail package define how the user addresses are formatted and how the message header is created. Each mail package will have adopted a particular standard as its default. In some cases this could be a proprietary standard, more open systems might use MHS or another published standard. There are, in short, hundreds of mail standards that do little except to confuse and get in the way of the idea of simple, global connectivity.

In the following pages, I have described the main mail standards for both network and Internet applications. If you are only dealing with the Internet, then you should skip straight to the section on SMTP, X.400 and POP3. If your Web server will have to transfer mail to other network mail server applications on a LAN, you should look at all the standards you might meet them all as you install your system!