Outlook's Cached Exchange 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.
Even when Outlook is configured in Cached Exchange Mode, Outlook must contact the server directly to
do certain operations. These operations won't function when Outlook is not connected and can take
longer to complete on high-latency connections. These operations include the following:
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Working with Shared Folders that were not made available offline.
Retrieving Free/Busy information.
Setting, changing, or canceling an Out of Office message.
Accessing public folders that were not made available offline.
Retrieving rights to a rights-protected message.
Editing rules.
Retrieving MailTips.
Retrieving Policy Tips.
Delayed delivery options are client side in cached mode. So, when you use Cached Exchange Mode,
Outlook must be connected and open at the assigned delivery time for the delayed delivery message to
be sent.
Outlook data file (.ost) recommendations
When you use Cached Exchange Mode, be aware that users' local .ost files are 50 percent to 80 percent
larger than the mailbox size reported in Exchange Server. The format that Outlook uses to store data
locally for Cached Exchange Mode is less space-efficient than the server data file format.
Managing performance issues in Outlook
Most users find that Cached Exchange Mode performs faster than online mode. However, several
factors can influence a user's perception of Cached Exchange Mode performance, like hard disk size and
speed, CPU speed, .ost file size, and the expected level of performance.
Managing Outlook folder sharing
By default, when Cached Exchange Mode is enabled, shared mail and nonmail folders that users access
in other mailboxes are downloaded and cached in the user's local .ost file. Similarly, if a manager
delegates access to his or her Inbox to a team member, when the team member accesses the folder,
Outlook 2016 also starts caching the Inbox folder locally.
Known issues with Cached Exchange Mode
Issue
Synchronization delays and
issues with shared mailboxes,
including shared calendars and
mail folders.
Only a subset of your Exchange
mailbox items are synchronized
in Outlook.
Description
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.
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
Resolution
We can disable caching of
additional mailboxes through
group policies.
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.
to view more on Microsoft
Exchange."
This setting does not affect the
number of items that are
synchronized with folders of the
following types:
Calendar
Contacts
Tasks
Journal
Notes
Outbox
Performance issues when there
are too many items or folders in
a Cached mode offline data
(.ost) file.
When you use Cached Exchange
Mode 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.
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.
For more, see Only a subset of
your Exchange mailbox items
are synchronized in Outlook.
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 are:
100,000 items per folder
500 folders total
If you have folders that exceed
the limit for the number of
items in 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
file.
To send a message immediately,
click the Send/Receive tab, and
then click Send All.