Post Office Protocol (POP3) Clients

Post Office Protocol version 3 (POP3) enables a client to retrieve mail from a remote server mailbox. A user who is not always attached to their network can have their mailbox on a server that is permanently attached to the network. Mail sent to that user would be delivered to the server-based mailbox, and the server would act as a sort of mail drop. Clients can remotely connect to the server and download their mail to their computer. The protocol used both to store the mail on the server and to download the mail to the client is POP3. In this capacity, the server is referred to as a POP server and the client as a POP client. POP3 cannot be used to send mail; it is only a retrieval protocol. Simple Mail Transfer Protocol (SMTP) is still used to transfer mail between mailboxes.

The remainder of this section covers these POP- related topics:

POP3 architecture
An overview of Exchange Server POP3 (including server and mailbox configuration)
POP3 client configuration

POP3 Architecture

POP works through a simple request-response mechanism. A POP client sends request commands, and a POP server sends responses back to the POP client. These client-server interactions can be divided into three main states:

Authorization
Transaction
Update

Authorization, also called greeting, is the client logon to the POP server. The POP username and password are sent to the POP server. After a successful authorization, transactions can take place between the POP client and server. The POP client can request the number and size of messages in its mailbox, and messages can be downloaded and deleted. After the POP server has responded and the POP client is finished, the POP client issues a Quit command. This ends the POP session and causes the POP server to enter the update state for the user ‚ s mailbox. Messages can be deleted during the update state.

POP uses TCP/IP as its transport protocol. The session, or conversation , between the POP client and server takes place on TCP port 110. If Secure Sockets Layer (SSL) is being used to create an encrypted channel, port 995 is used instead. A port is a numeric identifier assigned to an application or protocol and is used to route incoming packets to the correct application. Although a packet has arrived at the correct computer, it still has to be delivered to the correct application on that computer. POP clients address the requests to port 110 on the POP server. The POP server listens to port 110 for those requests (this same principle is applicable to LDAP, HTTP, and other Internet protocols). The third revision of the POP standard, POP3, is documented in RFC 1939.

Note ‚

You may be familiar with Ethernet or Token Ring addresses, which are used to deliver a frame to a specific computer. You may even be familiar with network addresses, such as IP addresses, which are used to route packets to the correct networks and computers. Ports are yet another type of address that is used to route packets to the correct applications on a machine.

Connecting Clients to Exchange Server 2003

The decision of how you will allow remote and non-Windows clients to connect to your Exchange Server 2003 messaging infrastructure is not an easy one, nor is it one that you should take lightly or make without giving due consideration to the benefits and drawbacks of everything involved. Will you use Outlook, taking advantage of the new RPC over HTTP feature, will you use the improved Outlook Web Access in Exchange Server 2003, or will you force these users to use an industry standard POP3 or IMAP4 application?

Several factors should influence your decision:

Which operating systems are in use by your clients?
Which operating systems are in use by your Exchange servers, domain controllers, and global catalog servers?
Are there existing means to securely connect users remotely over the Internet, such as VPN servers and so forth?
What is the company ‚ s willingness to try new, cutting-edge features?

These questions are just a few of the many that you must take into consideration when determining how you will allow remote and non-Windows clients to connect to the Exchange organization. As an example of how you might solve this problem, consider the following:
You have several Unix and Linux hosts on your network that will connect to the Exchange organization using Outlook Web Access.
You have some traveling salespeople who use Windows XP Professional computers. These users require full Outlook functionality and will connect using RPC over HTTP.
You have some other users who frequently travel and need access only to their Inboxes. When they travel, they keep all messages on their local computers in PST files. If needed, they move any messages back into their Exchange mailbox when they return to the company. These users connect to the Exchange organization using POP3 or IMAP4.

Exchange Server POP3 Overview

POP3 is integrated into the Internet Information Services (IIS) component of Windows Server 2003. Although it is not a separate service, the POP3 functionality is sometimes referred to as the POP3 Service. It permits any POP3-enabled e-mail program to connect to an Exchange server via IIS and retrieve mail (see Figure 7.22). Only messages in the Inbox folder of a mailbox can be accessed. The POP3 Service does not permit access to encrypted messages.

Figure 7.22: Exchange and POP3

As mentioned previously, POP3 retrieves mail but does not send it. The Simple Mail Transfer Protocol (SMTP) is used to send mail. SMTP functionality is also provided by IIS and uses TCP port 25.

POP3 Server Configuration

The configuration options available for a POP3 virtual server in Exchange Server 2003 are described in Table 7.5.

POP3 Mailbox Configuration

An administrator can override server-level settings at the mailbox level using the Protocols button on the Exchange Advanced page of a mailbox object. A mailbox can independently have POP3 enabled or disabled or have unique POP3 settings. As mentioned earlier, if a protocol is disabled at the server level, the settings at the mailbox level have no effect.

Table 7.5: Property Pages for the Site and Server POP3 Objects
Property Page	Description
General	This page is used to assign an IP address to the virtual server. The default is for the server to have access to all IP interfaces configured on the server. You can select a specific IP address or use the Advanced button to configure IP address and TCP port information. This page is used to configure how long an idle POP3 connection will be held open before automatically closing and to limit the number of connections that the virtual server will allow.
Access	The Authentication section of this page is used to select the authentication protocols that POP3 clients must use to log on to the Exchange server with the POP protocol. The options are Basic, which works using an unencrypted username and password, and Windows Integrated Authentication, which works using Windows Server 2003 network security and an encrypted password. The Secure Communication section of the Access page is used to configure a certificate server to provide POP3 security. This type of security is discussed in Chapter 15. The Connection Control section of the Access page is used to grant or deny access to the POP3 virtual server based on computer names or IP addresses.
Message Format	This page sets the encoding method and character set used when converting Exchange messages for retrieval by POP3 clients. The encoding options include MIME and UUENCODE. Selecting UUENCODE makes available the option to use Binhex with Macintosh clients. The default character set used is US-ASCII, but you can choose from many international sets. You can also specify whether Microsoft Exchange rich-text format can be used in POP3 messages.
Calendaring	The options on this page are used to configure URL access to calendaring information by POP3 clients. This allows POP3 clients to use OWA as their calendaring client.

Configuring a POP3 Client

The following information must be configured on a POP3 client in order for it to connect to a POP3 server (in this case, the POP3 server is an Exchange server with the Exchange POP3 Service enabled):

POP3 server name The computer name of the home server of the Exchange mailbox.

SMTP server name The computer name of the Exchange server that is supporting SMTP.

Note ‚

Unless the client is connected to a server on the same network, you may need to indicate a Fully Qualified Domain Name for the POP3 and SMTP server names.

POP3 account name The name the POP3 client must use when being authorized by a POP3 server. The Exchange POP3 Service requires a Windows Server 2003 domain and user account that has read permissions on the Exchange mailbox, followed by the alias name of the mailbox in the format domain\account\alias . If the account name and alias are the same, the alias name can be left off.

POP3 account password The Exchange POP3 Service requires the password of the Windows Server 2003 user account that is specified in the POP3 Account Name field.

POP3 client e-mail address The SMTP address of the POP3 client. For an Exchange mailbox, this is the SMTP address found on the E-mail Addresses property page of the mailbox.

Table 7.6 provides sample information and shows how that information can be used to configure a POP3 client. Exercise 7.2 outlines the steps for configuring Outlook Express as a POP3 client. This exercise assumes that Outlook Express is already installed on your system.

Table 7.6: An Example of a POP3 Client Configuration
Sample Information	POP3 Client Configuration
Computer name running the POP3 Service=Education	POP3 server name=Education
Computer name running SMTP=Education	SMTP server name=Education
Window Server 2003 account with read permission on the mailbox=Domain\GeorgeW Alias name of mailbox=GeorgeW	POP3 account name=Domain\GeorgeW\ GeorgeW or simply Domain\GeorgeW
Password of GeorgeW=woodenteeth	POP3 accountpassword=woodenteeth
Domain name=Chicago.com	POP3 client e-mail address=GeorgeW@
Alias name of mailbox=GeorgeW	Chicago.com

EXERCISE 7.2: Configuring Outlook Express As a POP3 Client

Click Start > Programs > Outlook Express.
Under the Tools menu, select the Accounts command.
Click the Add button and then click the menu ‚ s Mail command.
Enter a display name to appear in the From field of messages you send, and then click Next to continue.
Enter an e-mail address in the E-mail Address field and click Next.
Select the POP3 protocol from the drop-down menu.
In the Incoming Mail field, enter the name of the Exchange server that will service POP3 requests.
In the Outgoing Mail field, enter the name of the Exchange server that will service SMTP requests.
Click Next to go on.
In the Account Name field, enter the username of the mailbox to which you will connect.
Enter a password for the user, and then click Next.
Click Finish.