How Outlook for Windows connected to Exchange Online utilizes Microsoft Search
Published Sep 29 2020 07:18 AM 41K Views

Microsoft Search

In Microsoft 365, Microsoft Search returns search results based on the context of the application that you are searching from. When searching in Outlook, Microsoft Search is used to search email messages that are in your mailbox. There are plans for later builds to also query the Calendar, files, and other items within Outlook.


For general information on Microsoft Search, see Overview of Microsoft Search.


The evolution of Search in Outlook

It’s hard to imagine an Outlook without search. In its earliest versions, Outlook supported basic filters based on MAPI restrictions. Starting with Outlook 2007, Outlook has utilized Windows Desktop Search (WDS) to index Outlook Data (.pst) and Offline Outlook Data (.ost) files. These data files are normally stored on the local disk, as are the WDS search indexes. Although WDS provides excellent performance using the indexes on the local disk, mailbox sizes have increased while less mailbox data is synchronized to the local disk. This shift has resulted in a clear need for server- and service-powered search solutions.


This is where Microsoft Search comes into the picture. It fully indexes and searches content, while providing rich and intelligent search features that have become an industry norm. Throughout this journey, Outlook has evolved from local and MAPI-restriction searches, to server-based Exchange content indexing, to EWS/FAST search, and now to the most recent service-based iteration, Microsoft Search.


FAST Search continues to be used in Exchange Server 2016 and 2019 versions. For more information about FAST Search, see How Outlook 2016 utilizes Exchange Server 2016 FAST Search


Scenarios where Outlook uses Microsoft Search

This section is organized to help you determine which search solution is being used, based on a specific scenario. The Outlook product group is updating current versions of Outlook desktop, web, and mobile clients to migrate more and more scenarios to use Microsoft Search.


Note If Outlook cannot connect to Microsoft Search, an error message is displayed with a link to perform a local search.


The following tables list Outlook for Windows search scenarios, along with their current status and future plans to onboard them onto Microsoft Search.


Supported on Microsoft Search




Primary Mailbox - Cached Mode


Primary Mailbox - Online Mode (i.e., not Cached Mode)

Available in Monthly Channel builds >= 16.0.12716  

Note By default, new Exchange email accounts are configured in Cached Exchange Mode. Online Mode is not related to Internet connection status. 

[Explicitly Added] Shared Mailbox


All Mailboxes scoped search

Complete, available in builds >= 16.0.12716

Subfolder scoped search 

Available in Monthly Channel builds >= 16.0.12920 

Primary Calendar search 


Online Archive 

Available in Monthly Channel and Monthly Enterprise Channel builds >= 16.0.13519  

KQL Terms

Partially migrated - From, To, CC, Subject, HasAttachments, IsRead, Importance, Kind, and Size use Microsoft Search

Received as of build 16.0.14314

All other special KQL terms continue to rely on local search\MAPI restrictions

Note: New date picker in Advanced Search will leverage Microsoft Search



Migration to Microsoft Search In Progress

The Current Channel (Preview) referenced in the following table was formerly called Monthly Channel (Targeted) or Insider Slow. For details on the Current Channel Preview release dates, see




Microsoft 365 auto-expanding archive

Available in Monthly Enterprise Channel builds >= 16.0.13519

Note Not supported with on-premises primary mailbox. In a hybrid deployment, the mailbox must be an online mailbox configured with a cloud-based archive mailbox.

[Automapped] Shared Mailbox

In progress - Not yet available

[Automapped] Delegate Access

In progress - Not yet available

[Explicitly Added] Delegate Access

In progress - Not yet available

Shared Folder

In progress - Not yet available

Shared Calendar

Not started - Local search uses Windows Search. Online search uses MAPI restrictions 

Delegate Access Calendar

Not started - Local search uses Windows Search. Online search uses MAPI restrictions 



Not planned for migration to Microsoft Search




Public Folder

No plans to migrate - Local search uses WDS. Online search uses MAPI restrictions.

All Outlook Items Search

No plans to migrate - Local search uses WDS. Online search uses MAPI restrictions.

Group Search

No plans to migrate - Local WDS search only.  

Work Offline

No plans to migrate - By nature of the scenario, only local search is supported.


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Last update:
‎Jul 14 2023 09:03 AM
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