OUTLOOK OST FILE SIZE LIMIT ISSUE

Your 47.5-GB OST has reached Outlook’s default WarnLargeFileSize threshold, so permanently remove unneeded cached content and manually compact the OST; changing the synchronization period alone does not necessarily shrink the physical file.

At this threshold, Outlook can stop adding data and sending messages when sent mail must be stored in Sent Items. The default warning threshold documented for Outlook 2010, 2013, and 2016 is 48,640 MB (47.5 GB), with an absolute default maximum of 50 GB.^1^

1. Free space and compact the OST

Since reducing the synchronization period did not resolve it:

1. In classic Outlook, select File > Tools > Mailbox Cleanup.
2. Use View Mailbox Size to identify the largest folders.
3. Empty Deleted Items and remove unnecessary large messages or attachments. In Outlook Search, this query finds messages larger than 5 MB:

      messagesize:>5 mb
   

4. Permanently empty deleted content so that it becomes reclaimable space.
5. Select File > Account Settings > Account Settings > Data Files.
6. Select the affected .ost file, then choose Settings.
7. For an Exchange or Outlook.com account, select Advanced > Outlook Data File Settings.
8. Select Compact Now, wait for it to finish, and restart Outlook.

Outlook normally compacts during idle time only when the file contains more than 20% unused space; Compact Now starts that process immediately.^2^

Confirm afterward that the OST is comfortably below 47.5 GB and test both sending and receiving.

2. If compaction does not reduce it, rebuild the OST

Do this only after confirming that the required mail, calendar, and folders are visible in Outlook on the web or another server-connected client. An Exchange OST is a local copy of server data and can be recreated, but this process cannot recover content that exists only locally.

Microsoft documents the following recreation procedure:^3^

1. Exit Outlook.
2. Open Control Panel, search for Mail, and open it.
3. Select E-mail Accounts or Show Profiles > Properties > E-mail Accounts.
4. Select the Exchange account and choose Change.
5. Clear Use Cached Exchange Mode.
6. Select More Settings > Advanced > Offline Folder File Settings.
7. Select Disable Offline Use, then confirm.
8. Open Offline Folder File Settings again and select OK, then confirm.
9. Re-enable Use Cached Exchange Mode.
10. Select OK > Next > Finish, then close the Mail dialogs.
11. Start Outlook and allow the new OST to synchronize completely.

Keep the reduced Mail to keep offline period when rebuilding; the newly generated OST should then contain only the configured cached range rather than retaining the previous 47.5-GB physical allocation.

Conditional alternatives

• If the OST cannot be rebuilt or Outlook still reports the limit with a new OST: create a new Outlook profile through Control Panel > User Accounts > Mail > Show Profiles > Add, add the account, and select Prompt for a profile to be used. A new profile isolates corruption or profile-specific configuration.^4^
• Do not increase the registry limit as the first remedy. Microsoft permits Unicode limits to be raised but does not recommend doing so because performance can decrease. OST files of 25 GB or more already experience more frequent pauses.^5^
• For a managed business device: ask the Microsoft 365 administrator to check whether Outlook size-limit registry values are enforced through policy and whether downloaded shared folders are contributing to the OST. Shared-folder content can be stored in the same local OST.^1^

References

1. Configure the size limit for both .pst and .ost files in Outlook
2. Reduce the size of your mailbox and Outlook Data Files (.pst and .ost) | Microsoft Support
3. Open or import items from an offline Outlook Data File (.ost) | Microsoft Support
4. Classic Outlook not responding, stuck at "Processing," stopped working, or freezes | Microsoft Support
5. How to troubleshoot performance issues in Outlook | Microsoft Support