Enterprise J2ME Developing Mobile Java Applications [Electronic resources] نسخه متنی

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Enterprise J2ME Developing Mobile Java Applications [Electronic resources] - نسخه متنی

Michael Juntao Yuan

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8.1 Basics of Email


When you send an email to a friend, it looks as though the message goes directly from your mail program to your friend's. In reality, the message is handled by many servers that reside in the Internet infrastructure. Those servers talk to each other using standard protocols defined by groups such as IETF and W3C. For users, this process is entirely transparent. But developers have to understand exactly how an email message is transmitted to its destiny. In this section, we briefly go over important concepts and protocols in email.

The life cycle of an email message is as follows:


The sender's mail program contacts an SMTP (Simple Mail Transport Protocol) server. It tells the SMTP server the recipient and the content of the message.

The SMTP server finds the message recipient's mail server through an Internet registry and delivers the message to that server. The recipient server could be a POP3 (Post Office Protocol) or an IMAP (Internet Message Access Protocol) server.

The recipient POP3 or IMAP server stores the message. The next time the recipient logs onto that server, she will see that message.


The above process is illustrated in Figure 8.1. All data communication goes through TCP/IP sockets. SMTP, POP3, or IMAP servers are typically provided by ISPs or companies that have reliable Internet connections.


Figure 8.1. Internet mail architecture.


As application developers, we only need to care about how the user client interacts with the mail servers (i.e., the first and third steps).


8.1.1 The SMTP Server


SMTP servers that do not require user authentication are called open relays. Open relays deliver all messages received. However, due to the abuse of spammers, most SMTP servers today require authentication. Your SMTP username and password are normally the same as the ones for your main ISP account.


8.1.2 The POP3/IMAP Server


A POP3 server has only one inbox for each user. You should first download all email and then organize it using your mail client program. An IMAP server can provide multiple remote folders for each user. Those remote folders act as central message repositories and simplify email management when the user uses multiple mail client programs.


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