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Fix "The Messaging Interface Has Returned an Unknown Error" in Outlook

· 2 min read
Luke Murray
Luke Murray
Author

If you see the error "The messaging interface has returned an unknown error" when using Microsoft Outlook, it typically means your Outlook data file (PST or OST) has reached its maximum file size limit. Older versions of Outlook (2003 and earlier) had a default limit of around 2 GB, while Outlook 2007 and later support up to 50 GB, but the table format may still need upgrading.

How to fix it​

  1. Launch Microsoft Outlook.
  2. In the left-hand pane, right-click the folder list (your mailbox name) and select Properties.
  3. Click Advanced.
  4. Check the box labelled "Allow upgrade to large tables".
  5. Click Ok twice to close the dialogs.
  6. Restart Outlook for the change to take effect.

What does "Allow upgrade to large tables" do?​

Outlook data files use internal database tables to organise your emails, contacts, and calendar items. The "large tables" option upgrades the internal format to support a larger maximum file size and more items per folder. This one-time upgrade is non-reversible but is safe and recommended.

Additional steps to prevent the error​

  • Permanently delete old emails: Select emails you no longer need and press Shift + Delete to bypass the Deleted Items folder and immediately free up space in the data file.
  • Empty the Deleted Items folder: Right-click the Deleted Items folder and select Empty Folder.
  • Compact the data file: Go to File > Account Settings > Data Files, select your data file, and click Settings > Compact Now. This reclaims space from deleted items.
  • Archive old mail: Use File > Cleanup Tools > Archive to move older emails to a separate archive file, reducing the size of your primary data file.