About Outlook's Cached Exchange Mode and Online Mode

About Outlook's Exchange connection modes

Microsoft's Windows desktop versions of Outlook support two types of connectivity modes for Exchange accounts: Cached Exchange Mode and Online Mode.


Cached Exchange Mode

Cached Exchange Mode is the default and recommended configuration for most scenarios except those mentioned under When to use Online Mode.

When an Exchange account in Outlook is configured to use Cached Exchange Mode, Outlook works from a local copy of the user's Exchange mailbox that is stored in an offline data file (.ost file) on the user's computer, together with an Offline Address Book (OAB).

Cached Exchange Mode's purpose is to provide a seamless online and offline Outlook experience. It's also designed to help insulate users from network latency and connectivity issues while they are using Outlook.

By caching users' mailboxes to their computers, Outlook no longer depends on continuous network connectivity for access to user information. It also reduces network load. After messages have been pulled over the network one time, subsequent access to those messages does not cause additional network traffic.

If a user is offline and using Outlook, the program works from this local copy and with the Offline Address Book (OAB). Any email messages the user drafted while offline are automatically sent when that user is back online. When the user is online, the cached mailbox and OAB are periodically updated from the Exchange Server in the background.

Cached Exchange Mode is especially valuable for:

  • Portable computer users who frequently move in and out of connectivity.
  • Users who frequently work offline or without connectivity.
  • Users who have high-latency connections (greater than 500 ms) to the Exchange servers.

While Cached Exchange Mode is the default and recommended mode, there are some known issues with this mode, including synchronization delays with shared items and address book updates; see Known issues with Cached Exchange Mode.

Online Mode

The second type of Exchange connection mode is Online Mode. Online Mode works by using information directly from the server; as the name implies, it requires a connection. Mailbox data is only cached in memory and never written to disk. You can enable Online Mode by disabling Cached Exchange Mode.

When to use Online Mode

At IU, Online Mode is recommended for the following scenarios:

  • Kiosk scenarios, where a particular computer has many users who access different Outlook accounts, and the delay to download email messages to a local cache is unacceptable.
  • Large mailboxes on computers that don't have sufficient hard disk space for a local copy of the mailbox.
  • Large mailboxes (greater than 25 GB). For large mailboxes, performance can become an issue in Cached Exchange Mode.

Configuring the modes

By default, Cached Exchange Mode will be selected when a user adds an Exchange account in Outlook. To disable Cached Exchange Mode and switch to Online Mode, the user can choose to uncheck Use Cached Exchange Mode during account setup or by changing the Exchange account settings later within Outlook's Account Settings window.

To turn on Cached Exchange Mode in Office 365 or Office 2019:

  1. In Outlook, go to File, then Account Settings, and then Account Settings....
  2. Select your account, and then click Change....
  3. Check the Use Cached Exchange Mode to download email to an Outlook data file box, position the slider for email to download as desired (slide it all the way to the right to download all email), and then click Next.
  4. Click Done, and then close the "Account Settings" window.
  5. Restart Outlook.

For more details, see Turn on Cached Exchange Mode.

If you're a local UITS support person planning a deployment of Office, you can also configure Outlook's Exchange connection mode options by using the Office Customization Tool (OCT) or Group Policy. For more, see Plan and configure Cached Exchange Mode in Outlook 2016 for Windows.

Known issues with Cached Exchange Mode

Issue Description Resolution
Synchronization delays and issues with shared mailboxes, including shared calendars and mail folders.
By default, if an Outlook profile is configured with an Exchange account using Cached Exchange Mode and you add an additional Exchange mailbox to the profile, all items in all the folders to which you have access in the additional mailbox are downloaded to your offline Outlook data (.ost) file). These shared secondary mailboxes being cached can cause synchronization delays when delegates make updates to shared calendar items (including creating/editing meeting requests and appointments) and mail folders.

You can disable caching of additional mailboxes by following these steps:

  1. In Outlook, open the Account Settings window.
  2. Select the primary Exchange account, and click the Change button.
  3. Click the More Settings... button.
  4. Click the Advanced tab, and uncheck Download shared folders.
  5. Click OK, and then Next and Finish to close the Account Settings windows.

Shared mail folder caching can also be disabled using the Office Customization Tool (OCT) or Group Policy; see Shared mail folders are downloaded in Cached mode by default in Outlook 2010 and later versions.

Outlook for Mac versions experience synchronization delays with shared folders and calendar items.
Outlook for Mac desktop versions (including 2016) support only Cached Exchange Mode. Because the clients operate in this mode, this sometimes causes synchronization delays between the client and the Exchange server.
For Outlook 2016 for Mac, you can try to resolve the issue by resetting the cache for the unsynchronized folder or calendar; see Clear the cache in Outlook for Mac.
Only a subset of your Exchange mailbox items are synchronized in Outlook 2016.

The email folders for these mailboxes may show item counts that are lower than expected. Additionally, older items seem to be missing, and you receive the following message at the bottom of the list of items:

"There are more items in this folder on the server. Click here to view more on Microsoft Exchange."

This behavior occurs because the Cached Exchange mode Mail to keep offline setting is configured to a value other than All.

In the default configuration, depending on the size of your hard disk, Outlook synchronizes only 1, 3, or 12 months of email to your offline data (.ost) file from the Exchange server.

This setting does not affect the number of items that are synchronized with folders of the following types:

  • Calendar
  • Contacts
  • Tasks
  • Journal
  • Notes
  • Outbox

For more, see Only a subset of your Exchange mailbox items are synchronized in Outlook.

Performance issues when there are too many items or folders in a Cached mode offline data (.ost) file or Outlook data (.pst) file folder.

When you use Cached Exchange Mode or an Outlook data (.pst) file and you have a lot of items in any single folder, you may experience symptoms like the following in Outlook:

  • Decreased performance when switching into and out of a specific folder that has a lot of items.
  • Decreased performance in Outlook if the Inbox, Calendar, Tasks, Sent Items, and Deleted Items folders contain a lot of items.
  • Folders are not displayed correctly, or they take a long time to appear, especially in Cached Exchange Mode.

This problem may occur if you have folders that exceed the limit for the number of items per folder or if you have too many folders total. Item and folder limits for Outlook 2016 are:

  • 100,000 items per folder
  • 500 folders total

If you have folders that exceed the limit for the number of items in an Outlook data (.pst) file or an offline Outlook data (.ost) file, move items from the larger folders to separate or smaller folders in the same mailbox or data file; see Outlook performance issues when there are too many items or folders in a Cached mode .ost or .pst file folder.

Email messages aren't immediately sent.
When you are using Cached Exchange Mode, a message may remain in the Outbox for up to one minute until the next synchronization with the Exchange server occurs.

To send a message immediately, click the Send/Receive tab, and then click Send All.

For more, see Plan and configure Cached Exchange Mode in Outlook 2016 for Windows.

This is document adni in the Knowledge Base.
Last modified on 2024-04-15 16:43:14.