Outlook for Windows performs poorly (Cached Exchange Mode setup)
Microsoft has a documented limitation with Cached Exchange Mode with the number of items and folders that can be supported, which can cause poor performance of Outlook for Windows. These limits are:
- 100,000 items in a single folder
- 500 folders total
These limits are across all open accounts, including shared inboxes. More details can be found here:
https://support.microsoft.com/en-us/help/2768656/outlook-performance-issues-when-there-are-too-many-items-or-folders-in
Currently, there is no way to resolve this except to turn off cached exchange mode. This can be done for either all inboxes, or only shared inboxes. The following steps will disable cached exchange mode for shared inboxes only and leave the main account still running on cached exchange mode.
Problems you may encounter
- Outlook does not complete synchronization of account data.
- Old Outlook reminders appear.
- Outlook runs very slow.
- You can't access some information (Shared Calendars, Inboxes, Distribution Lists, etc.).
- Email or other data in your account is missing.
- Outlook freezes when sending messages and/or displays the error of 'Not Responding'.
Environment
Solution
- Open Outlook.
- Click on File -> Info -> Account Settings -> Account Settings
- On the Account Settings Screen select the person's email account and click 'Change'.
- In 'Server Settings':
- The 'Use Cached Exchange Mode' should be checked (check if not).
- "Mail to keep offline" should be set to 'All'.
- Click on the "More Settings" button. Select the 'Advanced Tab'. UNcheck the box for 'Download Shared Folders'.
- Click Apply and then OK. Then, close all windows and then restart Outlook.
Once Outlook has restarted, the Shared inboxes will no longer be operating under Cached Exchange mode and will be directly accessing those inboxes.
If Outlook continues to run slow after completing the above steps
The person's .ost file could potentially still be very bloated and this may still cause outlook to run poorly. In this case you can delete the .ost file. This will delete the locally cached copy of the emails. Outlook will be forced to re-download all the messages. While there is no clear threshold of what is 'too big', in general anything over ~5gb is fairly large. This can be accomplished by following these steps:
- Close Outlook
- Open this folder:
C:\Users\*Username*\AppData\Local\Microsoft\Outlook
- Delete the file ending in .ost.
NOTE: You may have to turn on file extensions, but it should be the largest file in the directory by far.
- Open Outlook and make sure messages start appearing.
Please note that these steps will force outlook to re-index the inbox so searching may not work for a while. It will also take some time for the full inbox to re-download, but generally not more than 10 to 15 min or so to download but this will vary depending on the size of the inbox.