Tài liệu SMTP and POP3 doc

5

SMTP and POP3: Communicating with

email Servers

5.1 Introduction

More emails are sent every day than postal mail. Why? Because email is

cheap, informal, fast, and can be picked up at the receiver’s convenience. Emails

can be automatically generated and sent, making them ideal for automated

status notification. One day, you may receive an email from your home sprinkler

system saying simply, “Your house is on fire.”

After reading this chapter you will be able to send and receive emails from your

.NET applications. These features can be useful for customer support systems,

collaborative personnel management, and many other types of applications.

This chapter begins by describing how emails can be constructed and sent, using

either a socket-level approach, or by using in-built .NET classes. Immediately

following that, is a description on how emails may be received, again, by either using

a socket level approach, or a higher-level methodol- ogy, leveraging Microsoft

Outlook.

5.2 Sending an email

Every email must have a destination email address. An email address takes

the following form:

<Username>@<domain name>

The domain name in an email address generally does not include the

“www” prefix, which is common for Web site addresses. Despite that, the domain

name is globally recognized under the DNS system. The username

is recognized only by the recipient mail server.

Emails are not immediately delivered to the recipient; instead, they are

initially sent to your ISP’s or company’s mail server. From there, they are forwarded

to the recipient’s mail server or held for a period of time until the recipient’s mail

server accepts the email. Emails are sent using the simple mail transfer protocol

(SMTP), which is described in detail later.

In order to determine the recipient’s mail server, a DNS mail exchange

(MX) query is issued to the local DNS server for that domain name. That computer

will then return details of where the server or servers that handle incoming mail are

located.

Note: Most ISPs have only one incoming mail server, but Hotmail.com has

more than 10 mail servers.

You will always be told the IP address of your SMTP server. Unfortu￾nately, you cannot use an SMTP server from another ISP because it will block you

with an error message such as “Relaying denied.”

Microsoft virtual SMTP server is available for most versions of Win- dows and

generally appears under IIS when installed.

5.2 SMTP

SMTP is used to send, but not receive, emails. Every mail server in the

world must conform to the SMTP standard in order to send emails reli- ably

regardless of destination. The definitive guide to SMTP is held by the Internet

Engineering Task Force (IETF) under RFC 821 at www.ietf.org/ rfc/rfc0821.txt.

The definitive guides to most major protocols are held at the IETF. They

are free to download and should be consulted when you are develop- ing network

applications that are designed to work with preexisting or third-party clients or

servers.

SMTP is not a difficult protocol to implement from the ground up;

however, it is natively supported from .NET and, thus, would be a waste of time to

redevelop. Also, many commercial email components are available, which can be

imported into your application. One of the most popular is AspEmail from Persits

Software. The demo version of this component is adequate for most applications.

5.3.1 Implementing SMTP

SMTP operates on TCP port 25. Before sitting down to code, you should

first find out the IP address of your ISP’s SMTP server. In the examples below, the

SMTP server smtp.ntlworld.com is used. You should replace this with your

own SMTP server, or the examples will not work.

SMTP was originally designed for UNIX users and has a command- line￾type feel to it, although the commands are issued over a network con- nection, rather

than a keyboard.

A good way to test the protocol is to open telnet by clicking Start→Run and type

telnet. In Windows NT, 2000, and XP, type o smtp.ntl￾world.com 25. In prior versions of Windows, click File→Connect, and then

type smtp.ntlworld.com into the connection box and 25 into the port box.

Then press Connect.

Once the client establishes a TCP connection to the server on port 25, the server

will always reply with 220 <some greeting message><enter>. A number is

always included at the start of every server response. Any number beginning with 5 is

an error and should be dealt with; everything else can