iPhone ActiveSync Feature Limitations
Although Apple has implemented a number of Exchange functions on the iPhone, it has not included all the features found in Outlook or on Windows Mobile devices. As mentioned earlier, the iPhone will sync a user's Inbox, calendar items, and personal contacts using direct push and ActiveSync. It will not sync tasks created in Outlook, provide management of personal or public folders available in Outlook, support the opening of links to Microsoft SharePoint server sites, let users set out-of-office autoreplies, create meeting invitations using the Calendar application, or support flagging of messages (such as for later follow-up).
It is also worth noting that at this point, direct push notification and sync occur for new e-mails only if they are delivered to a user's in-box. If users create filtering rules in Outlook that filter incoming mail into other mailboxes, the iPhone will not receive push notification of their delivery (though opening the mailbox in the iPhone's Mail application will cause it to be synced manually) because only the in-box is monitored. As a result, users should either remove such rules or configure them to be run manually when they are at their computer.
As I mentioned earlier, the iPhone can be a rather picky device when it comes to getting it working with Exchange. The following is a list of common issues that prevent the iPhone from being able to reliably access or sync Exchange accounts. This isn't a complete list of all known problems, but being aware of the most likely problems and their causes should help ensure a smoother iPhone implementation.
One potential cause for problems with iPhone/Exchange access is certificate management and SSL. As noted earlier, the iPhone prefers SSL and will attempt to connect to Exchange using SSL during setup as well as when sending ping requests. Microsoft suggests using SSL for all mobile devices with Exchange (which relies largely on HTTP/HTTPs as a communications protocol) because it ensures that casual sniffing of packets will not easily identify ping or sync requests for Exchange.
If you are using SSL, however, it is important that the certificate being used to sign communications is either installed on the iPhone or is signed by a certificate authority trusted by the iPhone. If a certificate cannot be verified, users will receive alerts to that affect when attempting to configure access to an Exchange account and when accessing the account. The inability to verify a certificate may also lead to additional connection and sync problems. Although disabling the use of SSL might appear to be one solution, it raises serious security concerns, particularly if users are connecting via unsecured Wi-Fi networks (which there is no feasible way to prevent).
Internal and External DNS
One of the challenges that the iPhone presents is that it can connect to network resources using a variety of mechanisms: a carrier's mobile network, a Wi-Fi network within your organization, or external Wi-Fi hot spots or home networks. Depending on how DNS and namespaces are implemented in your network, DNS lookups for the name of your Exchange server(s) may return different IP addresses when iPhone users are connected to an internal Wi-Fi network and when they attempt to connect from external Wi-Fi networks or via a carrier's mobile network. (This doesn't typically present a problem for mobile devices that rely solely on a carrier's network, since they will rely on external DNS servers for lookups and thus always receive IP addresses.)
This can result in situations where users can interact with Exchange while at work but not at other times. To avoid this problem, you can either use a VPN configuration on the iPhone or ensure that the DNS records accessed from the iPhone routinely receive an external IP address for your Exchange server(s).This may require review of your Exchange configuration as well as your overall network planning and perimeter devices (firewalls, ISA servers, etc.).
Needed Ports and Front-End/Back-End Server Configuration
Exchange communication requires configuration of appropriate ports for computers and devices that are outside your network. You should ensure that you have configured ports to allow traffic and to forward that traffic to the appropriate server(s). As an additional layer of security when configuring mobile device access, Microsoft recommends using Windows ISA Server and Exchange front-end and back-end servers (in which devices outside your network communicate only with the front-end server and not directly with the server that processes internal transactions). Refer to the Microsoft documentation listed at the end of this article for additional details on all of these configuration variables.
You will also need to verify that all network devices, such as routers, firewalls and other security appliances, that will process communication between your Exchange servers and iPhones outside your network are configured with timeout limitations that will not interfere with the heartbeat interval used for direct push. Using too-short timeouts for network communication devices could result in overall notification and sync failures for mobile devices, including the iPhone.