Summary

If Outlook stops receiving new emails, the local data file (.ost or .pst) may have reached Microsoft's 50 GB size limit. This article walks you through checking your mailbox size, reducing the sync window from 12 months to 6 months, and compacting the data file to reclaim space.

Before You Start

• Microsoft Outlook 365 on Windows.
• The account must be connected and showing Connected to: Microsoft Exchange in the status bar.

Instructions

1. Check Your Mailbox Size

Determine how large your local data file has grown.

1. Open Outlook.
2. Click File in the top-left corner.
3. The Account Information page opens. Under the mailbox name, look for the Mailbox Settings area.
4. Click Tools > Mailbox Cleanup.
5. Click View Mailbox Size….
6. A dialog box appears showing the total size of your mailbox and a breakdown per folder.
7. Note the Total Size value at the top. If it is 49 GB or higher, proceed to Method 2 below. If the total is well under that, size is not what is stopping your mail — for the other causes, see How to Fix Outlook Emails Not Syncing.

2. Reduce the Sync Window to 6 Months

Lowering the cached time range tells Outlook to remove older emails from the local file while keeping them available online.

1. Click File > Account Settings > Account Settings.
2. On the Email tab, select your Exchange account and click Change.
3. Locate the Mail to keep offline slider.
4. Drag the slider from 12 Months (or All) to 6 Months.
5. Click Next, then Done.
6. Outlook will begin removing items older than 6 months from the local file. This process runs in the background and may take several minutes depending on mailbox size.

Verification: After the sync completes, repeat Method 1 to confirm the total size has decreased.

3. Compact the Outlook Data File

After reducing synced content, the .ost file does not shrink automatically. Compacting reclaims the freed space on disk.

1. Click File > Account Settings > Account Settings.
2. Switch to the Data Files tab.
3. Select your data file (typically the one marked with a checkmark or the default .ost file).
4. Click Settings.
5. In the dialog that opens, click Compact Now.
6. A progress bar will appear. Let it finish — this can take several minutes for large files.
7. Click OK to close all dialogs.

Verification: Open File Explorer and navigate to the data file path shown on the Data Files tab (usually C:\Users\[YourName]\AppData\Local\Microsoft\Outlook\). Confirm the .ost file size has dropped significantly.

Troubleshooting

DANGER

Reducing the sync window does not delete emails from the server. Older emails remain accessible through Outlook on the Web (outlook.office.com) or by temporarily changing the slider back. No data is lost. To move older mail into separate server-side storage that does not count against your mailbox quota at all, see How to Archive Old Emails in Outlook.

Symptom / Error	Potential Cause	Solution
Mailbox size unchanged after reducing slider	Sync still in progress	Wait 15-30 minutes, then recheck via Mailbox Cleanup.
"Compact Now" button is greyed out	Outlook is still syncing	Close and reopen Outlook, then retry from the Data Files tab.
Emails still not arriving after compacting	Server-side mailbox also full	Contact IT to check your Exchange Online quota.
Outlook is extremely slow during compaction	Large file being rewritten	Let the process complete. Avoid closing Outlook mid-compact.

Related Articles

• How to Fix Outlook Emails Not Syncing
• How to Fix Outlook Not Opening or Crashing

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