Outlook 365 and Group mailboxes

Question

Good morning

We are using Outlook 365 and group mailboxes in the sales department. See below.

Microsoft® Outlook® for Microsoft 365 MSO (Version 2604 Build 16.0.19929.20172) 64-bit

We have cash mode enabled for 1 year. At some point, we are seeing users having issues with Outlook not responding or constantly syncing. Rebuilding profiles every other week. Is there another way to fix this besides rebuilding Outlook profiles?

Answer 1

Good morning

I did a test on one system and removed the cached mode, and it was very slow to access. cache is now set to 6 months and constantly syncing.

Answer 2

Hello @James Sigmon

I would like to kindly clarify that as a user-to-user forum moderator, I do not have access or authority to make changes directly to your account or environment. The suggestions I provide are based on experience, research, and similar cases observed within the community, and I will do my best to support you.

Based on your description, the first thing that comes to mind is how Outlook handles shared mailboxes in Cached Mode. By default, Outlook downloads all shared mailbox content into the local OST file. If those shared mailboxes are large or very active (which is common in sales environments), the OST file can grow significantly and lead to decreased performance, especially symptoms like constant syncing or Outlook becoming unresponsive.

In addition, when there are a large number of items or folders in the mailbox, Outlook performance can degrade noticeably, and synchronization issues may occur.

To help improve the situation, you may consider reducing the Cached Mode sync window from 1 year down to around 3 - 6 months. This might reduce the amount of data stored locally and lower the syncing load.

Another option is to disable caching for shared mail folders (uncheck “Download shared folders”), which prevents Outlook from storing shared mailbox data in the local OST file and can improve performance in many shared mailbox scenarios.

It may also be helpful to review the size and item count of the shared mailboxes. If any folders contain a very large number of items, applying archiving or retention policies can reduce the load and improve performance.

You may also want to take a look at the following articles for more insights:

• https://learn.microsoft.com/en-us/microsoft-365-apps/outlook/data-files/shared-mail-folders-in-cached-exchange-mode
• https://learn.microsoft.com/en-us/troubleshoot/outlook/performance/performance-issues-if-too-many-items-or-folders

Hope this info helpful.

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Answer 3

We have run repairs on the office many times with no end. Ongoing issue.

Answer 4

The described behavior (Outlook not responding, constant syncing, frequent profile rebuilds) matches known Outlook desktop issues that are not resolved by repeatedly recreating profiles.

From the available information, there are two main patterns in similar Outlook issues:

1. Outlook crashes, hangs, or stops responding
   • Recommended remediation is to repair the Office installation rather than continually rebuilding profiles.
   • Steps:
     1. Open Control Panel in Windows.
     2. Select Uninstall a program under Programs.
     3. In the list of installed programs, right-click the Microsoft 365 Apps/Office 365 installation.
     4. Choose Change.
     5. Select Online Repair and complete the repair.
   • This repairs the Office binaries and can resolve stability and responsiveness issues that affect Outlook, including hangs and freezes.
2. Profile-related or mailbox-related corruption
   • When Outlook behavior is tied to specific mailboxes or profiles (for example, only some users or only certain mailboxes), the guidance is to adjust or clean up profiles rather than constantly recreating them from scratch.
   • For persistent profile problems, the documented approach is:
     • Remove only problematic or extra profiles and keep the primary working profile.
     • Steps (for reference when profiles are clearly implicated):
       1. Close Outlook.
       2. Open Control Panel → Mail → Show Profiles.
       3. Remove unneeded or problematic profiles, keeping the primary profile.
       4. If necessary, create a new profile and add the accounts again.
     • Note that removing a profile deletes its cached data files; ensure data is on the server or backed up before removal.

Given that multiple users are affected and profiles are being rebuilt frequently, a more sustainable approach is:

1. Run an Online Repair of Microsoft 365 Apps on affected machines instead of repeatedly recreating Outlook profiles.
2. Use Mail in Control Panel to remove only clearly problematic profiles, not to rebuild on a schedule.
3. After repair, monitor whether Outlook still hangs or constantly syncs; if it does, investigate environmental factors (add-ins, security software, or third-party tools) and consider temporarily disabling them to isolate the cause.

These steps provide a supported alternative to constant profile rebuilds and directly target Outlook/Office installation integrity and profile health.

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References:

• Outlook crashes or stops responding when used with Office 365
• "Your mailbox has been temporarily moved on Microsoft Exchange server” message when an Office 365 user opens Outlook