What Is POP3 Service?

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

What Is POP3 Service?

In this section

  • Common POP3 Scenarios

  • Other E-Mail Retrieval Technologies

  • POP3 Dependencies

  • Related Information

The POP3 service supplements existing functionality in Windows Server 2003 provided by the SMTP service, which receives e-mail messages. The POP3 service makes e-mail messages available for download from a server, enabling a server to host e-mail accounts and provide basic e-mail access.

The POP3 service performs the tasks of message download and request handling on a Windows-based server, where message download consists of transmitting the messages from a folder in the file system to clients and request handling is performed according to the POP3 protocol, which defines how the server responds to requests sent from an e-mail client.

Basic e-mail access depends on the POP3 protocol implemented by the POP3 service. The POP3 protocol, described in RFC 1939, “Post Office Protocol - Version 3,” a standard for retrieving e-mail messages from servers, is stable, widely used, and supports basic e-mail access. The POP3 protocol supports message download to a client after the Simple Mail Transfer Protocol (SMTP) delivers messages received by means of the Internet. SMTP transmits e-mail from the originating server to the destination server using a protocol defined in the proposed standard, RFC 2821, “Simple Mail Transfer Protocol.” For more information about the POP3 and SMTP protocols, see RFC 1393 and RFC 2821 in the IETF RFC Database.

The POP3 service is a scaleable e-mail server that meets the basic e-mail access needs of businesses and organizations. Any client that supports RFC 1939 can be configured to use the POP3 service to download e-mail. The POP3 service supports the following client authentication methods: Active Directory integrated authentication, Local Windows accounts authentication, and Encrypted Password file authentication. Because the POP3 service mail store exists in the file system, message storage and retrieval do not require specific database technologies on the server.

Common POP3 Scenarios

The POP3 service is designed to meet the needs of businesses and organizations with a small number of users needing basic e-mail access. However, the POP3 service is capable of scaling up to meet the needs of Internet service providers (ISPs) offering basic e-mail access to a large number of users.

Small Businesses and Organizations

Many small businesses have an investment in a Windows Server infrastructure that is suitable for an e-mail platform like POP3. POP3 enables organizations to host e-mail services for their Internet domain and support basic e-mail service. E-mail clients receive e-mail from the POP3 server and send e-mail by using the SMTP service. Groupware and scheduling functionality are not available with the POP3 service.

Internet Service Providers

ISPs offer e-mail service to customers as an added incentive; therefore the cost of e-mail service must be low enough that it does not impact the bottom line. Because POP3 use is so widespread, system administrators are familiar with existing POP3-compatible technology. The POP3 service in Windows Server 2003 supports a variety of authentication methods and scales up well to meet the demands of the ISP. Additionally, ISPs with large user bases can take advantage of the Active Directory directory service to store account information.

Other E-Mail Retrieval Technologies

POP3 is not the only protocol to support message retrieval. Internet Message Application Protocol version 4 (IMAP4) is similar in purpose to POP3. The IMAP4 protocol defines an extensive command set, including message searching, downloading of individual message body parts, and message tagging. IMAP4 implementations are not as common as POP3 because of the complexity of the IMAP4 protocol. Consequently, the use of IMAP4 is not as widespread as POP3 although the use of IMAP4 is growing as users demand more features from ISPs. For more information about IMAP4, see RFC 3501, “Internet Message Access Protocol -Version 4, Rev 1” in the IETF RFC Database.

Messaging API (MAPI) is a protocol used with Microsoft Exchange Server and various e-mail clients for feature-rich messaging, including groupware and scheduling. One of the capabilities of MAPI is message retrieval for clients, such as Microsoft Outlook 2003. MAPI provides access to messages and folders and the associated properties of messaging objects. Message retrieval occurs in the context of a remote procedure call (RPC) from the client to the Exchange Server store, with the message contents transmitted in the response to the RPC call.

POP3 Dependencies

The POP3 service has the following dependencies:

  • Authentication Methods. Active Directory integrated authentication, Local Windows accounts authentication, or Encrypted Password file authentication are the authentication methods that may be used with the POP3 service. All methods allow plaintext authentication, permitting credentials to be transmitted without encryption. If Active Directory-integrated authentication is chosen, Active Directory must be in place and functioning properly.

  • NTFS or FAT file system. The POP3 service uses the file system as the mail store and takes advantage of NTFS access control lists (ACLs) to secure the mailboxes. POP3 service supports other file systems but NTFS provides the access control features that secure the mailboxes, and is strongly recommended.

  • Mail delivery from the Internet. The SMTP service is a component that is installed by the POP3 service and receives incoming mail for the POP3 service.

The following resources contain additional information that is relevant to this section.

  • NTFS Technical Reference

  • RFC 1939, “Post Office Protocol - Version 3” in the IETF RFC Database

  • RFC 2821, “Simple Mail Transfer Protocol” in the IETF RFC Database

  • RFC 3501, “Internet Message Access Protocol -Version 4, Rev 1” in the IETF RFC Database