How to Check and Manage Mailbox Size in Microsoft Outlook

When an Exchange or Microsoft 365 mailbox approaches its designated storage limit, Outlook begins issuing warnings and may eventually block incoming and outgoing email transmission. While modern enterprise quotas often reach 50 GB or 100 GB, massive PDF attachments, recorded Teams meetings, and archived PST files can exhaust storage faster than anticipated. Microsoft Outlook includes built-in diagnostic and management utilities that allow you to inspect per-folder storage consumption and automate mailbox cleanups.

How to Check Mailbox Storage Consumption in Outlook

You can view your active account size and storage capacity directly from the backstage view:

1. Open Microsoft Outlook on your computer.
2. Click the File tab in the upper-left corner of the Ribbon.
3. In the Account Information screen, locate the Mailbox Settings section.
4. Outlook presents a visual storage progress bar displaying total consumed space and remaining headroom (e.g. 42.8 GB free of 49.5 GB).

Auditing Storage by Individual Folder Size

If your quota is nearing exhaustion, inspecting which specific folders contain the heaviest burden helps target remediation:

1. On the File > Info screen, click the Tools button located beside the Mailbox Settings bar.
2. Select Mailbox Cleanup… from the dropdown menu.
3. In the Mailbox Cleanup window, click View Mailbox Size…
4. The Folder Size dialogue opens, displaying two insightful tabs:
   • Local Data: Reflects the size of items cached locally on your device in your offline Outlook data file (.ost).
   • Server Data: Displays the exact storage footprints of each folder stored on the remote Exchange/Microsoft 365 server, including Hidden Items, Conversation History, and Calendar attachments.
5. Examine the folder breakdown to identify whether Sent Items, Deleted Items, or specific archival folders are driving storage exhaustion.

Using Mailbox Cleanup to Find and Purge Bulky Emails

The Mailbox Cleanup dialogue box features rapid query tools to identify large messages without requiring complex manual search syntax:

1. Within the Mailbox Cleanup dialogue, select Find items larger than and set a threshold (for example, 10000 KB / 10 MB).
2. Click Find… to launch Advanced Find filtered specifically for heavy messages.
3. Sort the results list by the Size column header.
4. Inspect the listed messages: download and save necessary large file attachments to OneDrive or local storage, then delete the emails from Outlook.

Automating Deleted Items and Junk Purges

Emails moved to the Deleted Items folder still count against your active mailbox storage quota until permanently emptied:

• Empty Deleted Items Folder: In the Mailbox Cleanup dialogue, click Empty to purge the folder permanently.
• View Deleted Items Size: Click View Deleted Items Size… to assess how many megabytes will be reclaimed before committing the purge.
• Automate on Exit: To ensure discarded messages do not accumulate silently, go to File > Options > Advanced, and under Outlook start and exit, tick the box for Empty Deleted Items folders when exiting Outlook.