Outlook mailbox space can fill up faster than expected, but a few targeted actions will free it up significantly. Knowing how to reduce outlook mailbox size is essential for anyone who relies on email daily. A bloated mailbox slows down performance, causes sync errors, and can even prevent you from sending or receiving new messages. This guide walks you through every practical method to reclaim space, from quick cleanups to long-term strategies.
Why Your Outlook Mailbox Gets So Large
Before diving into solutions, it helps to understand the main culprits. Attachments are the biggest space hogs—large PDFs, images, and presentations eat up megabytes fast. Deleted items and sent items folders often hold onto copies you forgot about. Calendar entries, tasks, and notes also take up room, though less dramatically. Over time, these accumulate without you noticing.
Another common issue is the “Sync Issues” folder, which can balloon with duplicate items. And if you use Outlook with Exchange or Office 365, your mailbox has a storage limit—typically 50 GB for business accounts. Hitting that limit means no more email until you clear space.
Check Your Current Mailbox Size First
You need to know how much space you’re using before you can reduce it. Here’s how to check:
- Open Outlook and go to File > Account Settings > Account Settings.
- Select your email account and click Change.
- Look for “Mailbox Size” or “More Settings” > “General” > “Mailbox Size.”
- Alternatively, right-click your mailbox folder in the folder pane, choose Data File Properties, then Folder Size.
This shows total size and breakdown by folder. Note the largest folders—those are your targets.
How To Reduce Outlook Mailbox Size: Step-By-Step Methods
Now for the core actions. Each method below tackles a different source of bloat. Combine them for maximum impact.
Empty The Deleted Items Folder Regularly
This is the simplest fix. Outlook doesn’t automatically purge deleted items unless you set it to. Over weeks, this folder can hold thousands of messages.
- Right-click Deleted Items in the folder pane.
- Choose Empty Folder.
- Confirm the deletion.
Do this weekly. For automatic cleanup: go to File > Options > Advanced, then under “Outlook start and exit,” check “Empty Deleted Items folders when exiting Outlook.”
Clear The Sent Items Folder
Every email you send stays in Sent Items. If you send large attachments, this folder grows fast. You don’t need to keep every sent message forever.
- Open Sent Items.
- Sort by size (click the “Size” column header).
- Delete emails with large attachments you no longer need.
- Or archive old sent items to a PST file (see archiving section below).
Remove Large Attachments From All Folders
Attachments are the number one space consumer. Outlook has a built-in tool to find them.
- Click in the search box at the top of your mailbox.
- Type hasattachments:yes and press Enter.
- Sort results by size (click “Size” column).
- Delete emails with attachments you don’t need, or save attachments to your hard drive and delete the email.
For a more thorough search, use size:>5 MB to find emails over 5 MB. Delete or archive them.
Use The Mailbox Cleanup Tool
Outlook includes a built-in cleanup utility. It’s underused but very effective.
- Go to File > Tools > Mailbox Cleanup.
- Click Find items larger than and set a size (e.g., 500 KB).
- Review the list and delete or move items.
- Also use AutoArchive settings to move old items to a PST file automatically.
Archive Old Emails To A PST File
Archiving moves older emails out of your main mailbox into a separate data file. This keeps your mailbox lean without losing data.
- Go to File > Account Settings > Account Settings.
- Click the Data Files tab.
- Click Add to create a new Outlook Data File (.pst). Name it “Archive.”
- Now, manually drag folders or emails into the new PST file.
Alternatively, set up AutoArchive: File > Options > Advanced > AutoArchive Settings. Choose to archive items older than 6 months, and move them to your PST file.
Compact Your PST Or OST File
When you delete items, Outlook doesn’t immediately shrink the data file. The space is marked as available but not released. Compacting reclaims that space.
- Go to File > Account Settings > Account Settings.
- Click the Data Files tab.
- Select your mailbox file (usually .ost for Exchange, .pst for POP3).
- Click Settings > Compact Now.
This can take a few minutes. Do it after major cleanup.
Reduce Calendar And Task Items
Calendar entries, especially recurring ones with attachments, take space. Old tasks and notes also add up.
- Delete old calendar items: Switch to Calendar view, go to a past year, and delete or archive old appointments.
- Remove tasks you’ve completed and no longer need.
- Clear the Notes folder of old notes.
Disable Cached Exchange Mode (For Business Users)
If you use Exchange or Office 365, Outlook downloads a copy of your mailbox locally (OST file). This can be huge. Disabling cached mode reduces local storage.
- Go to File > Account Settings > Account Settings.
- Select your Exchange account and click Change.
- Uncheck Use Cached Exchange Mode.
- Restart Outlook.
Note: This means slower performance when offline, but it saves disk space.
Use Online Mode Or Reduce Sync Slider
If you need cached mode, you can limit how much mail is synced. In Outlook 2016 and later, go to Send/Receive > Work Offline settings, or adjust the slider in account settings to sync only 1 month or 3 months of email.
Delete Sync Issues Folder Contents
This hidden folder can hold duplicate items and errors. It’s safe to clear.
- In the folder pane, scroll down to find Sync Issues (may be under your mailbox name).
- Open it and delete all items inside.
- Empty Deleted Items afterward.
Long-Term Strategies To Keep Mailbox Small
Cleaning once isn’t enough. Adopt these habits to prevent future bloat.
Set A Retention Policy
If your IT department allows, set up retention policies to auto-delete old emails. For personal accounts, use Outlook rules to move old messages to archive after 90 days.
Use Cloud Storage For Attachments
Instead of attaching files directly, upload them to OneDrive, Google Drive, or Dropbox and share a link. This keeps the email tiny.
Unsubscribe From Newsletters
Promotional emails with images and tracking pixels add up. Unsubscribe from ones you never read. Use a service like Unroll.Me to bulk unsubscribe.
Regularly Delete Junk And Clutter
Empty your Junk Email folder weekly. Also check the Clutter folder (if enabled) for low-priority messages you don’t need.
Use Search Folders For Cleanup
Create a search folder for “Large Messages” to find big emails quickly. Right-click Search Folders > New Search Folder > Large Messages.
Advanced Techniques For Stubborn Mailboxes
If standard methods aren’t enough, try these.
Export To PST And Start Fresh
Export your entire mailbox to a PST file, then delete everything from the server. Reimport only what you need.
- Go to File > Open & Export > Import/Export.
- Choose Export to a file > Outlook Data File (.pst).
- Select your mailbox and include subfolders. Save the file.
- After export, delete all items from your mailbox (be careful—backup first).
- Import back only essential folders.
Use Third-Party Tools
Tools like MailStore, Kernel for Outlook, or Stellar Outlook Toolkit can analyze and clean mailboxes. They offer features like deduplication and attachment extraction. Use with caution and backup first.
Contact IT For Server-Side Cleanup
If you’re on Exchange or Office 365, your admin can run server-side scripts to delete old items or enforce quotas. Ask them to enable “In-Place Archive” which moves old emails to a separate mailbox automatically.
Frequently Asked Questions
How often should I clean my Outlook mailbox?
At least once a month. Set a reminder to empty Deleted Items, clear Sent Items, and run the Mailbox Cleanup tool. Weekly quick cleanups are even better.
Will deleting emails from Outlook delete them from the server?
It depends on your account type. For IMAP and Exchange, yes—deleting in Outlook deletes from the server. For POP3, deletions are local only unless you configure otherwise.
Can I reduce mailbox size without deleting emails?
Yes. Archiving to a PST file removes emails from the server but keeps them accessible. Compacting the data file also frees space without deleting anything.
Why is my mailbox still large after deleting items?
Outlook doesn’t shrink the data file immediately. You need to compact the PST or OST file (see section above). Also check Sync Issues and Junk folders.
Does Outlook Web Access (OWA) have the same cleanup tools?
OWA has limited cleanup options. You can delete items and empty folders, but advanced tools like Mailbox Cleanup and AutoArchive are only in the desktop app.
Final Checklist For A Lean Mailbox
Follow this quick checklist after reading the article:
- Check your current mailbox size.
- Empty Deleted Items and Sent Items.
- Remove large attachments (search for size:>5 MB).
- Run Mailbox Cleanup tool.
- Archive old emails to PST.
- Compact the data file.
- Disable cached mode if needed.
- Set up AutoArchive for future.
- Unsubscribe from newsletters.
- Schedule monthly cleanup.
By applying these methods, you’ll not only reduce your Outlook mailbox size but also improve performance and avoid hitting storage limits. Start with the simplest steps—emptying folders and removing attachments—and work your way up to archiving and compaction. Your inbox will thank you.