Outlook Not Retrieving New Emails - Outlook Data File has Reached the Maximum Size Issue The Outlook Data File has reached the maximum size. Your mailbox may or may not be full in Exchange Online. Outlook will not retrieve any new emails. Error Microsoft Outlook The Outlook data file C:\Users\%username%\AppData\Local\Microsoft\Outlook\%useremail%.ost has reached the maximum size. To reduce the amount of data in this file, permanently delete some items that you no longer need. Cause This issue can be caused by your mailbox being full in Exchange Online, but more typically, it occurs whenever you have Read and Manage access to one or more other mailboxes while your Outlook is set to Cached Exchange Mode. Resolution Check Mailbox Size You can verify the mailbox size by opening the Outlook desktop app and clicking File at the top. In the Mailbox Settings section on the Info tab, you can see the current mailbox size. If the mailbox is close to 50 GB, then you will need to start with deleting or archiving as many emails as you can. Otherwise, if the mailbox has free space, then you can proceed with the next steps to reduce the Outlook Data File size: Disable Caching for Shared Mailboxes Most commonly, users have Read and Manage access to one or more shared mailboxes, and Outlook automatically downloads a copy of all emails for these mailboxes while Cached Exchange Mode is enabled. All emails are stored in the same Outlook Data File, causing it to fill up to the maximum size of 50 GB very quickly. To fix this, open the Outlook desktop app. Click File at the top. Click Account Settings and then Account Settings... again. Click Data Files and double-click your email in the list. Click the Advanced tab, uncheck the option to Download shared folders and click OK. A prompt will appear stating, "You must restart Outlook for these changes to take effect." Click OK. Right-click Outlook in the taskbar and click Close all windows. Save any open drafts if necessary. Open Outlook again. Outlook should no longer be downloading copies of emails in shared mailboxes, but you will likely need to clean up the Outlook Data File still. To do this, repeat Steps 6 through 8. Click the Advanced tab and then click Outlook Data File Settings… Click Compact Now. Outlook will be frozen and unusable for the 2 or 3 minutes it takes to complete. Once finished, click OK. The Outlook Data File should now be reduced in size, and Outlook should be working again.