E-mail concepts

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

E-mail concepts

Components of the POP3 e-mail system

The three components of the POP3 e-mail system are the POP3 e-mail client, the Simple Mail Transfer Protocol (SMTP) service, and the POP3 service. These are each described in the following table.

Component	Description
POP3 e-mail client	The POP3 e-mail client software that is used to read, compose, and manage e-mail. The POP3 e-mail client retrieves e-mail from the mail server and transfers it to the user's local computer so it can then be managed by the user. An example of an e-mail client that supports the POP3 protocol is Microsoft Outlook Express.
SMTP service	The e-mail transfer system that routes e-mail from the sender to the recipient using the SMTP protocol. The POP3 service uses the SMTP service as the e-mail transfer system. E-mail is composed by the user on a POP3 e-mail client. Then, after the user connects to the mail server using an Internet or network connection, the SMTP service picks up and transfers the e-mail across the Internet to the recipient's mail server.
POP3 service	The e-mail retrieval system that downloads e-mail from the mail server to the user's local computer using the POP3 protocol. The POP3 protocol controls the connection between a user's POP3 e-mail client and the server where e-mail is stored.

Administrators manage the POP3 service on three organizational levels: mail servers, e-mail domains, and mailboxes. These are described in the following table.

Category	Description
Mail server	The computer where the POP3 service is installed. Users connect to it to retrieve their e-mail.
E-mail domains	The e-mail domain must be a registered domain name and it must match the Mail eXchanger (MX) record created by your Internet Service Provider (ISP).
Mailboxes	Each mailbox corresponds to a user who is a member of the e-mail domain, such as someone@example.com. The user's mailbox corresponds to a directory in the mail store in which the user's e-mail is stored until it is retrieved.

A POP3 e-mail system

In the diagram, e-mail is sent to someone@example.com, as illustrated by step 1. The e-mail is picked up by the SMTP service and is sent out on the Internet (step 2).

The e-mail domain, example.com, is then resolved to a mail server on the Internet, mailserver1.example.com (step 3). Mailserver1.example.com is a mail server running the POP3 service that receives incoming e-mail for the e-mail domain, example.com.

In step 4, the e-mail for someone@example.com is received by mailserver1.example.com and is moved into the mail store directory (step 5), where e-mail intended for someone@example.com is stored.

In step 6, the user, someone, checks for e-mail by connecting to the mail server running the POP3 service. The POP3 protocol transmits the user and password authentication credentials for the user, someone. The POP3 service verifies these credentials, and then either accepts or denies the connection.

If the connection is accepted, all of the e-mail for the user, someone, that is being stored in the mail store is downloaded from the mail server to someone's local computer (step 7). The mail is usually then deleted from the mail store.

Network security

The POP3 and SMTP protocols are not encrypted. If someone gains access to the network on which the server running the POP3 service is running, they could potentially read users' e-mail.

To increase network security, implement Internet Protocol security (IPSec).

IPSec is a framework of open standards for ensuring private, secure communications over Internet Protocol (IP) networks through the use of cryptographic security services. For more information about IPSec, see Internet Protocol Security (IPSec).