How Outlook for Windows connected to Exchange Online utilizes Microsoft Search

Published Sep 29 2020 07:18 AM 10.4K 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


[Explicitly Added] Shared Mailbox


All Mailboxes scoped search

Complete, available in builds >= 16.0.12716

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




Primary Mailbox - Online Mode

(i.e. not Cached Mode)


Available in Monthly Channel builds >= 16.0.12716 

Note By default, accounts run in Cached Mode. Online Mode is not related to Internet connection status.

Online Archive

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

Microsoft 365 auto-expanding archive

Available in Monthly Channel builds >= 16.0.13519

[Explicitly Added] Delegate Access

Not working - Under investigation

Subfolder scoped search

Available in Monthly Channel builds >= 16.0.12920

Primary Calendar Search

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




Shared Folder

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

Public Folder

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

[Automapped] Shared Mailbox

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

[Automapped] Delegate Access

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.



This is gold, thank you. Hope you update the blog as additional scenarios are being addressed.

 (also a small typo for the Online Archive row :))


Hi @Jason Creighton
could you please explain more precisely what does it mean “[Explicitly Added] Shared Mailbox?". Btw.: the scope you have provided “All Mailbox" does not exist Search 1.JPG– see screen below. The client version is 16.0.13127.20620 64 bit.


@Red Flag , All Mailbox should have said All Mailboxes. Just updated the article and fixed the typo. Thanks. (Note: That scope only appears when you have multiple accounts added to Outlook)


The [Explicitly Added] note is to highlight where Microsoft Search works only for accounts that were added via File -> Add Account.

This is in contrast to automapped account scenario where the Shared or Delegate mailbox automatically shows up in Outlook after signing into your primary account.


Thanks a lot, @Jason Creighton for clarification. Adding a Shared Mailbox through File > Add Account requires a password - but fine. Background for my question: customers with large quantity of Shared Mailboxes and heavy e-mail workloads that require precise search accross all auto-mapped shared / delegated mailboxes complain about non-consistent search results while searching over all mailboxes. I hope, Microsoft Search could improve the results - however it would need to enable passwords on those shared mailboxes. From security point of view it's a less preferable scenario. As the article states - no plans for moving Shared Mailbox to Microsoft Search, no other choice... Thanks again.

New Contributor

@Jason Creighton, does this functionality change the way searching works for auto-expanding in-place archives? Are you able to search against the entire archive or are you still required to search folders individually?

Occasional Visitor

Good morning,


Just wanted to check in and see if there was any information on when Microsoft Search for Online Mode will get rolled out in the non preview channel? I can't find any relevant info in the roadmap. Thanks!


Primary Mailbox - Online Mode

(i.e. not Cached Mode)


Available in Current Channel (Preview) builds >= 16.0.12022 

Regular Contributor

@Jason Creighton 

M365 biz user here. When I use "Microsoft Search" to search "Work" in, it seems that does not include *any* Exchange online search results, yet practically every other type of M365 artifact is searched, even Teams convos.

Screenshot 2021-08-14 134846.gif


I was under the impression that Exchange email search results would be coming to this way of searching when I asked about it a couple years ago (2019 or thereabouts) but am still not seeing anything. What gives?


@Bob Manjoney. They are using Microsoft Search, but are still working on support for displaying email results. I know it's under experimentation but don't have an ETA at this time. Ultimately the team will need to look at the metrics and determine what value the email results are adding and the impact to other areas of the search results to determine a final ship verdict.

<Edit. Added some additional details> 

Occasional Visitor

For Office 365 if I select to search All Mailboxes does it search the subfolders in the mailboxes?


This is important for the way we have the shared mailbox setup, everything is in subfolders, the top level is empty.






Hi @RobHulley - based on my experience, the announcement above didn't change the behaviour for search when ShareMailbox has been added as usual and preferable = with auto-mapping. The search results for "All Mailboxes" are still incomplete and inconsistent. Only when when you highlight the particular Inbox for a given ShareMailbox and set the search scope to "Current folder" the search results are accurate. This scenario makes a search across ALL SharedMailboxes assigned to a user with auto-mapping NOT WORKING.

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