Understanding POP3 and IMAP4

Applies to: Exchange Server 2010

Topic Last Modified: 2009-11-17

By default, POP3 and IMAP4 are disabled in Microsoft Exchange Server 2010. To support clients that still rely on these protocols, you must first start the POP3 and IMAP4 services on the Exchange 2010 Client Access server. You must also configure SMTP for your POP3 and IMAP4 clients to send e-mail.

For detailed steps about how to enable the POP3 and IMAP4 services, see Enable POP3 in Exchange 2010 and Enable IMAP4 in Exchange 2010.

By default, users who have mailboxes on computers that are running Exchange 2010 can access their mailboxes by using Microsoft Outlook or Outlook Web App, Microsoft Exchange ActiveSync, or Outlook Voice Access. Outlook, Outlook Web App, and Outlook Voice Access enable your e-mail users to use the comprehensive set of features that are available to clients who have mailboxes on Exchange 2010 servers.


Overview of POP3 and IMAP4 Functionality

POP3 and IMAP4 Cross-Site Connectivity

Managing POP3 and IMAP4 with Exchange 2003

Using Non-Standard Accounts with POP3 and IMAP4

Understanding Differences Between POP3 and IMAP4

Send Receive Options for POP3 and IMAP4 E-Mail Applications

POP3 and IMAP4 Applications

User Settings to Configure POP3 or IMAP4 Access to Their Exchange 2010 Mailboxes

This section describes the POP3 and IMAP4 functionality for Exchange 2010.

These two protocols have the following benefits and limitations:

  • POP3   POP3 was designed to support offline mail processing. With POP3, e-mail messages are removed from the server and stored on the local POP3 client, unless the client has been set to leave mail on the server. This puts the data management and security responsibility in the hands of the user. POP3 doesn't offer advanced collaboration features such as calendaring, contacts, and tasks.
  • IMAP4   IMAP4 offers offline and online access, but like POP3, IMAP4 doesn't offer advanced collaboration features such as calendaring, contacts, and tasks.

POP3 and IMAP4 can't be used to send messages from a client application to the e-mail server. E-mail applications that use POP3 and IMAP4 to send messages rely on the SMTP protocol to send messages. The connector for receiving e-mail submissions from client applications that use POP3 or IMAP4 is created automatically on every Hub Transport server. For more information about connectors, see Understanding Receive Connectors.

In earlier versions of Exchange, you had to perform a manual configuration step to allow your POP3 and IMAP4 clients to connect to their mail from one site in your organization when their mailbox was located in a different site in your organization. By default, Exchange 2010 automatically proxies from a Client Access server in one site to the correct server.

When you deploy Client Access servers to support clients that use POP3 and IMAP4, and their mailboxes are located on Exchange Server 2003 back-end servers, you must use Basic authentication. Also, you won't be able to use Secure Sockets Layer (SSL) encryption. Instead, you must use Internet Protocol security (IPsec) to help secure the communication between these servers.

You can't use an Anonymous account or Guest account to sign in to an Exchange 2010 mailbox through POP3 or IMAP4. This kind of access is blocked because of security vulnerabilities when you use non-standard accounts for POP3 and IMAP4 access. Additionally, you can't connect to the Administrator mailbox through POP3 or IMAP4. This limitation was included intentionally in Exchange 2010 to enhance security for the Administrator mailbox. To access the Administrator mailbox, you must use Microsoft Office Outlook or Outlook Web App.

POP3 is a frequently used e-mail Internet protocol. By default, when POP3 e-mail applications download e-mail messages to a client computer, the downloaded messages are removed from the server. When a copy of your user's e-mail isn't kept on the e-mail server, the user can't access the same e-mail messages from multiple computers. However, some POP3 e-mail applications can be configured to keep copies of the messages on the server so that the same e-mail messages can be accessed from another computer. POP3 client applications can only be used to download messages from the e-mail server to a single folder (usually the Inbox) on the client computer. The POP3 protocol can't synchronize multiple folders on the e-mail server with multiple folders on the client computer. POP3 also doesn't support public folder access.

E-mail client applications that use IMAP4 are more flexible and generally offer more features than e-mail client applications that use POP3. By default, when IMAP4 e-mail applications download e-mail messages to a client computer, a copy of downloaded messages remains on the e-mail server. Because a copy of the user’s e-mail message is kept on the e-mail server, the user can access the same e-mail message from multiple computers. With IMAP4 e-mail, the user can access and create multiple e-mail folders on the e-mail server. Users can then access any of their messages on the server from computers in multiple locations. For example, most IMAP4 applications can be configured to keep a copy of a user's sent items on the server so that they can view their sent items from any other computer. IMAP4 supports additional features that are supported by most IMAP4 applications. For example, some IMAP4 applications include a feature that lets the user view only the headers of their e-mail messages on the server—who the message is from and the subject—and then download only the messages that they want to read. IMAP4 also supports public folder access.

IMAP4 and POP3 clients have limited access to calendar information for Exchange. For more information, see Configure Calendar Options for IMAP4 and Configure Calendar Options for POP3.

POP3 and IMAP4 e-mail applications let users choose when they want to connect to the server to send and receive e-mail. This section discusses some of the most common connectivity options and also provides some factors your users should consider when they select connection options available in their POP3 and IMAP4 e-mail applications.

Three of the most common connection settings that can be set on the POP3 or IMAP4 client application are:

  • To send and receive messages every time the e-mail application is started. When this option is used, mail is only sent and received upon starting the e-mail application.
  • To send and receive messages manually. When this option is used, messages are only sent and received when the user clicks a "send and receive" option in the client user interface.
  • To send and receive messages every set number of minutes. When this option is used, the client application connects to the server every set number of minutes to send messages and download any new messages.

For information about how to configure these settings for the e-mail application that you use, see the Help documentation that's provided with the respective e-mail application.

If the device or computer that's running the POP3 or IMAP4 e-mail application is always connected to the Internet, users may want to configure their e-mail application to send and receive messages every set number of minutes. Connecting to the server at frequent intervals lets the user keep their e-mail application up-to-date with the most current information on the server. However, if the device or computer that's running the POP3 or IMAP4 e-mail application isn't always connected to the Internet (for example, if the user connects to the Internet by using a dial-up connection), the user may want to configure the e-mail application to send and receive messages manually. In a dial-up connectivity scenario, sending and receiving messages manually can potentially reduce the time that a user is connected to the Internet.

If the user is using an IMAP4-compliant e-mail application that supports the IMAP4 IDLE command, the user may be able to send e-mail to and receive e-mail from their Exchange mailbox in near real time. For this connection method to work, both the e-mail server application and the client application must support the IMAP4 IDLE command. In most cases, users don't have to configure any settings in their IMAP4 application to use this connection method.

Because Exchange 2010 supports POP3 and IMAP4, users can use any applications that support POP3 and IMAP4 client applications to connect to Exchange 2010. These applications include Outlook, Windows Mail, Microsoft Outlook Express, Entourage, and many third-party applications such as Mozilla Thunderbird and Eudora. The features supported by each e-mail client applications vary. For information about the specific features offered by specific POP3 and IMAP4 client applications, see the documentation that's included with each application.

After you enable POP3 and IMAP4 client access on your Client Access servers, you have to give users the information they need to connect their e-mail programs to their Exchange 2010 mailbox. Users can find the information they need to set up their POP3 or IMAP4 program by opening the About page in the Outlook Web App Help. They'll need the following information:

  • POP3 or IMAP4 server name
  • POP3 or IMAP4 port number
  • POP3 or IMAP4 encryption method
  • SMTP server name
  • SMTP (outgoing server) port number
  • SMTP encryption method
For your users to access the SMTP server information, you must run the Set-ReceiveConnector cmdlet with the AdvertiseClientSettings parameter. For more information, see the description of the AdvertiseClientSettings parameter in the topic Set-ReceiveConnector.
The default setting on some e-mail programs is not to keep a copy of messages on the server after they are retrieved to the client. Be sure to recommend to your users that they make sure that they've set up their e-mail program to keep a copy of the messages that the client retrieves on the server. Keeping a copy of messages on the server enables your users to access their messages from a different mail program.

After your user signs in to their mailbox using Outlook Web App successfully, they must click the drop-down arrow next to the Help question mark, and then click About. The POP3, IMAP4, and SMTP information they need is located on that page.