I do a good share of my work on the road—last year, I worked more than 100 days in sites around the world. Wherever I am, I need to process the hundreds of messages that arrive in my mailbox every day, so I especially need good offline functionality. I've found that the combination of the Microsoft Exchange Server and Outlook client delivers what I need.

In Exchange, working offline means that you can use the client when you aren't connected to an Exchange server. You can configure server-based folders—the Inbox, Outbox, Sent Items, and Deleted Items—for offline use. Then you can dial in to your network and work remotely or synchronize your offline folders with the folders on the server. Based on my experience over the past two years, I've compiled some tips for working offline effectively.

Tip 1: Set Up OST, PST, PAB, and OAB
Four types of files are important for working offline:

  • The offline store (OST), which contains slave replicas of selected server-based folders—replicas of the Inbox, Outbox, and Sent Items folders and the other special folders (Calendar, Tasks, Journal, and so on) that Outlook maintains. You can have only one OST.
  • Personal stores (PSTs), which are local file stores that extend the overall storage capacity available to users. A client can open multiple personal stores during a session. Exchange doesn't replicate the folders in personal stores to or from server folders, as it does with an OST. Personal stores are optional for working offline.
  • The personal address book (PAB), which holds the email addresses of people you correspond with. Entries in Outlook's Contacts folder can also have email addresses that you can use to address messages.
  • The offline address book (OAB), which holds email addresses from the Exchange Directory. Exchange generates the default OAB from the Global Address List, but Exchange can generate different OABs based on Address Book Views. You can, for instance, have an OAB that contains the names only of people in a certain country or department. Outlook treats the OAB as one logical file but holds the data in several separate .oab files in the Windows directory. This arrangement can be confusing for users who roam from PC to PC, because they can easily forget to copy a set of .oab files to take between PCs. Exchange usually generates the OAB every night.

By default, Exchange stores the OST, PST, and PAB in the Windows directory. I recommend that you create another directory (e.g., C:\<username>\exchange) for these files. Using a different directory establishes a clear separation between Windows and application data and lets you maintain privacy more easily. I also recommend changing the name of your OST from the default (exchange.ost) to something more personal, such as <username>.ost, so you can more easily associate files with particular users. You can't move the .oab files outside the Windows directory.

Tip 2: Control OST Size
The size of an OST varies from user to user. My OST ranges between 40MB and 60MB. I keep replicas of some project folders, and some personal and public folders. My OST usually contains between 4000 and 6000 items across all folders.

Some people settle for a base set of special folders; others like to bring along server-based folders. User discipline is an important factor. An OST can hold a maximum of 2GB, 16,384 items in one folder, and an unlimited number of folders. The same limits apply to a PST. In Exchange 5.5, server-based stores have no limits, except for disk space or mailbox quotas. If you delete some messages immediately, you will store fewer messages than if you keep everything you receive. Smaller folders equal smaller OSTs. Outlook clients store more information in some synchronized folders (such as the Journal) that Exchange clients don't use, so if you use Outlook, your OST will be slightly larger.

Tip 3: Add a PST to Your User Profile
User profiles don't usually include PSTs. You must add a PST to your profile through Outlook's Tools, Services option. Click Add and Personal Folders, and then specify the file's location. If Outlook can't find the file in the specified location, it will offer to create a new PST. Screen 1 shows Outlook adding a new PST to a profile.

Tip 4: Decide Whether to Use PSTs
You don't need PSTs to work offline. However, people often use PSTs either as the default destination for messages or as a way to extend the available amount of storage. The latter use is less necessary with the advent of unlimited server storage in Exchange 5.5. Nevertheless, some people like having their data in files they control.

Tip 5: Use PSTs to Transfer Items Between Users and to Archive Items
You can use PSTs to transfer messages, documents, and other items between users. Using PSTs in this way is advantageous because it preserves all of an item's attributes and attachments. You can also transfer complete folders or sets of folders at one time. Using the procedure (shown in Tip 3) for creating a new PST, create a PST on a disk, and then drag the items from server folders to the PST. Now you can give the disk to anyone you need to share the data with. If the PST doesn't fit on a disk, you can compress the PST. You can also use PSTs as archives. Moving items out of server folders reduces the demand for storage and makes your offline stores smaller.

Tip 6: Use Both the PAB and the OAB
The PAB and OAB let Outlook address messages when you're working offline. Exchange doesn't validate addresses again when you connect to the server and send the messages you've created offline. If Outlook retrieves an invalid address from the PAB or OAB, you'll get a nondelivery notification and have to send the message again.

Putting email addresses into the PAB is easy, but the data remains personal. The PAB doesn't let you update addresses by comparing them with the Exchange Directory or any other authoritative source, and addresses tend to become obsolete. Addresses from the OAB are usually more accurate, largely because they come from the Exchange Directory, which systems administrators maintain. However, you can't put personal information in the Exchange directory, so most people work with a combination of a PAB and an OAB.

Tip 7: Use Differential Downloads
Downloading an OAB generated from a large directory can take a long time. Exchange 5.5 allows differential downloads: Exchange retrieves from the server only entries that administrators have changed, added, or deleted since the last download. This feature reduces download times by a huge factor. I can perform a weekly download in less than 1 minute; it previously took 15 minutes. People still using Exchange 4.0 or 5.0 will have to download the OAB across the LAN before they leave the office.

Tip 8: Synchronize Offline Replicas with Server-based Folders
Synchronization uploads and sends messages created on the PC, downloads newly received messages, and adjusts the content of folders to match the work you've done offline. For example, if you add a new item to an offline folder, Exchange automatically uploads that content to the server during synchronization.

To perform synchronization, select Tools, Synchronize. Choose This Folder to synchronize a selected folder marked for offline access, or choose All Folders to synchronize all marked folders. Screen 2 shows the result of choosing the All Folders option. Exchange also downloads views and the folder hierarchy and, as the first step in the synchronization process, delivers to the server any messages waiting to be sent.

After Outlook completes synchronization, it creates a log of the activity and places the log in the Deleted Items folder. The log lists the number of items that Outlook has changed, deleted, or added to each folder.

Tip 9: Take Public Folders on the Road
You can create replicas of public folders for offline access. Replicated public folders are an excellent way to take with you shared project material or information that you never get a chance to read in the office. I subscribed a public folder to the Internet mailing list for Exchange (msexchange@insite.co.uk), and I take a replica of this folder on my trips. I can then catch up on the information in the list when I have a quiet moment.

You must mark a public folder as a favorite before you can download its contents. Select the folder and choose File, Folder, Add to Public Folder Favorites to add it to your favorites. Then use the Folder, Properties option to mark the folder for offline access, as Screen 3 shows. Outlook doesn't synchronize public folders automatically, as it does with private folders. You must either use the Tools, Synchronize, All Folders option to synchronize all the private and public folders that you've marked for offline access, or select the folder and use the Tools, Synchronize, This Folder option to synchronize just the selected folder.

Tip 10: Be Prepared
Exchange makes working offline easy, but only if your installation offers a supporting infrastructure and users know what they're doing. A little training will help ensure that remote workers can function effectively.