MSO365: Corrupt OST File While exceeding Current Mailbox Size (Up to 50GB) of Microsoft Outlook
Microsoft Outlook uses an OST (Offline Storage Table) file to store cached mailbox data in Cached Exchange Mode. Although Outlook supports Local data as OST files up to 50 GB by default, corruption risk increases as the OST grows very large. Ultimately, Outlook starts having performance and synchronization issues, leading to the error message and preventing further synchronization of new data. In fact, Microsoft 365 Business Premium, or a Microsoft 365 Enterprise E3/E5 license (Source) to increase its storage limit to 100GB.
To resolve this issue, some probable instructions below :
Method 1 : Cached less amount of mail locally (Offline Mode).
Fig:01 - Account information from Outlook Client.
Ex: 3 months length would be max size of OST 22GB~25GB.
Fig-02 : choose “Decimal” and in the value data type the new file size in Megabytes (MB)
Method 2. Disable Caching of Shared folders.
Method 3: Utilize In-Place-Archive (Online Archive)
From the Exchange Admin Center (EAC) applying the Online archive automatically for each mail-address or Manually move to archive (which is not recommended). Archived emails are stored on the server, not the local .ost file, but are still accessible via Outlook and webmail.
Method 4: Increase Outlook PST file size from Windows registry. (May be/May not be)
1. RUN => regedit => HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\16.0\Outlook\PST
2. Create Two Files in PST folder a) MaxLargeFileSize b) WarnLargeFileSize
N:B: 1GB=1024MB, 30GB=30720MB, 50GB=51200MB, 70GB=71680MB, 100GB=102400MB
Approx. Estimate for WarnLargerFileSize = MaxLargeFileSize X 95%
Official References:
# Configure the size limit for both (.pst) and (.ost) files in Outlook (Reference of Method-4)
When perform above of this Scenario, there might be occurred following issue :
I am doing 6 months retention policy for my office365 mail account but for reducing overreaching size of the current mailbox, I'm configuring 3 months mailbox as the offline cached exchange mode. Now I want to see at the my current mailbox as per retention policy about 6 months, whereas I only get at the current mailbox for 3 months mail, and rest of 3 months mail can not see at the current mailbox. At the Online In-Place Archive, I just see mail from 7 months to onwards mail. Between 3 to 6 months mail can not see at the neither current mailbox nor In-place Archive.
How to fix this problem for office365 mailbox while configuring at the Outlook Desktop Client . .?
Solution:
This scenario occurs during used at the Desktop Outlook Client because your local Outlook cache (3 months) and your server-side retention/archive policy (6 months) are not aligned, creating a "gap" of missing data in the middle. As because your local cache is set to only download 3 months, they are not visible in your Outlook desktop app. But rest of 3 months mail will be exist at the main mailbox on the server. Approximate scenario would be configure by using below picture's concept.
To fix this so you can see all 6 months of data, follow these steps in order:
1. Fix the "Missing" Months (3-6 Months Range): Configure the Offline Cached Exchanged mode as per Retention policy at the Desktop Outlook Client Apps.
2. Adjust the Online-In-Place-Archive: Reviewing the Retention policy for being current mailbox by helping of the Office365 Administrator.
