Outlook PST is very large and not getting smaller after deleting emails?

I archived and deleted a bunch of email messages in Outlook, in order to reduce the size of the PST file containing them. However after doing so, the PST’s no smaller. What gives?

•There are two very common possibilities:

  1. there’s deleting, and then there’s DELETING
  2. and even after DELETING, a little patience is called for

Fortunately, there are ways to force things and bypass that whole pesky “patience thing”.

Much like Windows itself, your PST has a an equivalent to the Recycle Bin called the “Deleted Items” folder. By default, when you delete an email message (or just about anything that Outlook stores in your PST), it’s not deleted at all, but simply moved to the Deleted Items folder.

The whole point of Deleted Items is as a safety net; a way to recover from “oops, I didn’t mean to delete that” because things aren’t really deleted.

So if things aren’t really deleted, then of course no space will be freed up.

You can configure Outlook to empty the recycle bin when you exit Outlook, or you can simply empty it yourself. In the folder list, right click on the Deleted Items folder and click on Empty “Deleted Items” Folder:

Empty Deleted Items folder selection in Outlook

This will permanently delete the items from your PST.

However your PST won’t get smaller. At least, not right away.

“If you empty your Deleted Items folder, and just leave Outlook alone for a while, sometimes a long while, it will dutifully compact and shrink your PST.”

When Outlook actually deletes an email (i.e. when it does not put it in the Deleted Items folder, or when it removes it from that folder), the space taken up by that email is “freed”. In other words that space within the PST is marked as unused, and made available for other email messages that might arrive thereafter. So your PST won’t get smaller right away, but it’s also quite likely that it won’t get bigger as new mail arrives either. The space freed within the PST by emptying the recycle bin will simply get reused.

I keep saying “right away” because Outlook will start making your PST smaller, after some time.

If you leave Outlook running, after some amount of idle time, it will start to perform something called “compaction”. Compaction is very similar to defragmenting your hard disk. During compaction it moves all of the email messages and other “real” information in the PST to the front of the PST, and moves all the unused or free space within the PST to the back. When compaction is done, Outlook can then actually make the PST file smaller by removing that portion of the PST that isn’t actually being used by anything.

That’s where patience comes in. If you empty your Deleted Items folder, and just leave Outlook alone for a while, sometimes a long while, it will dutifully compact and shrink your PST.

If you’re impatient, like I am, you can force compaction.

In folder view, right click on Personal Folders:

Outlook Personal Folders context menu

And click on Properties for “Personal Folders” …. You’ll get this dialog:

Outlook Personal Folders Properties dialog

Click on the Advanced… button, and you’ll get this:

Outlook Personal Folders Properties Advanced dialog

And as you might guess, hit that Compact Now button. You’ll then see this small dialog for a while:

Compacting Now Status Dialog

Exactly how long will depend on the size of your PST, and how much free space there is within it. It can take quite a while for larger files.

When it’s done, your PST will be as small as it can be to hold what’s inside.

  • Windows Vista users: Follow my “Browsing websites gets painfully slow with Windows Vista” article. Running the following command might give you a nice performance boost, and not just related to Outlook 2007.
    • Click Start and type CMD.
    • Press CTRL+SHIFT+ENTER to open the command Prompt with Administrative rights.
    • At the prompt in the Administrator: Command Prompt window, type the following command, and then press ENTER:
      netsh int tcp set global autotuninglevel=disable
    • Reboot your computer for settings to apply.
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