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 doesn’t support public folder access.

Note:
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.

Common Configuration Settings

Considerations When Selecting Send Receive Options

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.

Note:

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. They'll need the following information:

To connect from inside the corporate network, users will need the following information:

• Internal POP3 or IMAP4 server name
  
  
• Internal POP3 or IMAP4 port number
  
  
• Internal POP3 or IMAP4 encryption method
  
  
• Internal SMTP (outgoing server) name
  
  
• Internal SMTP (outgoing server) port number
  
  
• Internal SMTP (outgoing server) encryption method
  
  

To connect from the Internet, they’ll need the following information:

• External POP3 or IMAP4 server name
  
  
• External POP3 or IMAP4 port number
  
  
• External POP3 or IMAP4 encryption method
  
  
• External SMTP (outgoing server) name
  
  
• External SMTP (outgoing server) port number
  
  
• External SMTP (outgoing server) encryption method
  
  

You can make these settings available to your users through e-mail or other manual communication methods. You can also configure Exchange so that your users can use Outlook Web App to look up their own settings.

Configuring Exchange So Users Can Look Up Their Internal POP3, IMAP4, and SMTP Server Settings

By default, your users can look up their internal POP3 and IMAP4 server settings through Outlook Web App. However, to allow your users to access internal SMTP (outgoing) server settings, you must run the Set-ReceiveConnector cmdlet with the AdvertiseClientSettings parameter. After you run this command, your users can access their internal POP, IMAP, and SMTP server settings through Outlook Web App by clicking the drop-down arrow next to the Help question mark, and then clicking About.

For detailed information about how to configure this setting, see Allow POP3, IMAP4, and SMTP Server Settings to be Viewed By End Users in Outlook Web App.

Configuring Exchange So Users Can Look Up Their External POP3, IMAP4, and SMTP Server Settings

By default, external POP3, IMAP4, and SMTP server settings aren't available to your users through Outlook Web App. You can change the default setting as follows:

• To allow your users to look up their own external POP3 settings, you must run the Set-POPSettings cmdlet with the ExternalConnectionSettings parameter.
  
  
• To allow your users to look up their own external IMAP4 settings, you must run the Set-IMAPSettings cmdlet with the ExternalConnectionSettings parameter.
  
  
• To allow your users to access external SMTP server settings, you must run the Set-ReceiveConnector cmdlet with the AdvertiseClientSettings parameter.
  
  

After you change your default settings by running the Set-POPSettings, Set-IMAPSettings, and Set-ReceiveConnector cmdlets, your users can look up their external POP, IMAP, and SMTP server settings in Outlook Web App as follows:

• If you're running Exchange Server 2010 SP1, your users can look up their settings in Outlook Web App by clicking Options > All Options > Account > My Account > Settings for POP, IMAP, and SMTP access.
  
  
• If you're running the RTM version of Exchange 2010, your users can look up their settings in Outlook Web App by clicking the drop-down arrow next to the Help question mark, and then clicking About.
  
  

For detailed information about how to configure this setting, see Allow POP3, IMAP4, and SMTP Server Settings to be Viewed By End Users in Outlook Web App.

Leaving a Copy of Messages on the Server

The default setting on some e-mail programs isn't to keep a copy of messages on the server after they're retrieved. Be sure to recommend that your users set up their e-mail program to keep a copy of all messages the client retrieves on the server. By keeping a copy of messages on the server, your users can access their messages from a different e-mail program.