One of my users has a huge mailbox (12Gb). This is causing issues with Outlook 2007 (which Outlook 2003 didn't have). He is a laptop user and is running Windows 7.

The laptop has 4Gb of ram, a 160Gb 7200rpm disk and a dual core 2.6Ghz processor.

Outlook has been installed with service pack 2. It's also configured in cached mode. The user needs to be able to use his email when offline.

I've spend a few hours googling this and have try the top few suggestions.

  1. Renamed
    "C:...\AppData\Local\Microsoft\Outlook\extend.dat" [to extend.old]
    "C:...\AppData\Local\Microsoft\FORMS\FRMCACHE.dat" [to frmCache.old]
  2. Scanned the OST file
  3. Created a new Outlook profile
  4. I also found the this KB article: http://support.microsoft.com/kb/933493 which describes my problem exactly. However the page doesn't mention support for windows 7. I tried it and I got an error installing the patch with "Expected version not found"
  5. I have checked the plugins that are run. There aren't any except the default ones found on a fresh install (this laptop is a new build with a fresh install of Office on it only)

Does anyone else have any ideas?

Ideally I can't remove attachments out of the email (or even filter them from syncing locally), I can't switch to a "live mode" profile and the user doesn't realy want to archive off his old e-mail as lots of it is still referenced. The user doesn't use Google Desktop and makes good use of the Windows 7 search features.




4 Answers 4


After having gone through this same issue with our Outlook clients after being upgraded to Outlook 2007 I can tell you first hand (and from my conversations with MS) that the problem is not the size of the mailbox or the size of the pst\ost file (MS said they regularly see 20GB mailboxes with no performance issues) but with the number of items in each mailbox folder. For the best performance the MS tech rep recommended having 1,000 or less items per mailbox folder. If the user isn't willling to archive any of his email, then my suggestion would be to create subfolders for any folder that has more than 1,000 items in it and move the bulk of the items to these subfolders. MS has published various recommendations over the years regarding the number of items per folder (generally in the 3,000 to 5,000 item range) but we've had the best results using 1,000 or less as the MS tech rep suggested. Below is a link to just one of the many articles discussing this issue.


  • Hmm, a smaller folder makes Outlook fetch less data from the file, nice idea... Jul 21, 2010 at 9:59
  • I shall read through your link and check his mailbox. Thanks Jul 22, 2010 at 8:09

He should store the attachments on the disk, they are not meant to be kept in Outlook (it's a mail client, not a file server). It's completely normal for an inbox of such size to cause performance problems, and to get rid of the size one only could move the attachments out of the file. An alternative solution if he really doesn't want to learn the proper way is to archive or export old and big mails to another PST/OST file.

  • Storing attachments on disk causes more issues. What happens when he uses webmail? How would he access his attachments? Also this issue ONLY came around after upgrading to Office 2007. So are you saying this is normal for Outlook in general? Or is this an issue with Outlook 2007 only? Maybe the only answer is to roll back to 2003 or try 2010 Jul 20, 2010 at 15:23
  • Why would storing attachments cause more issues? He can find them on his disk, or if he needs it at several places he could put it on a network or online share. The PST might have changed, 2003 might cause compatibility problems in the future (and disallows upgrading Exchange to a new and more secure version for backwards compatibility puproses) and 2010 might show the same occurence. Why not just solve the problem instead of looking for a lot of workarounds? The whole 12 GB is not gonna be referenced or needed in the next week... Jul 20, 2010 at 17:47
  • It's not so much the storing his files on his laptop but more the "what happens when he's using webmail from a random pc". Jul 22, 2010 at 8:08
  • He could put it on a network or online share or archive them into several folders. I don't understand why one must keep 12 GB in his inbox (the main mail folder)... Jul 22, 2010 at 9:45

I'll throw this one out there since nobody else has mentioned it: if you right-click on the main folder in Outlook and select Properties, then click Advanced, there should be a 'Compact Now' button. It may take a while to run, but it sometimes helps.

  • The problem is that a lot of attachments are stored in his inbox which causes a lot of internal fragmentation or a lot of disk reads (to access various parts of the big file) which causes Outlook to slow down, compacting is not going to help with that I'm afraid... Jul 20, 2010 at 17:49
  • @TomWij: I thought that one of the things compacting does is to alleviate fragmentation within the file. Mainly it's to remove empty space, thereby reducing the size of the file, of course, and that in itself could help.
    – boot13
    Jul 21, 2010 at 1:06
  • Hmm, that can be right... But I'm wondering how the attachments are stored in the file, if they sit together then there shouldn't be performance issues, but if they are spreak over it then there could be serious problems. Compacting will indeed get some space away but not enough to cope with the disk having to handle 10+ GB. Even if there wasn't fragmentation on the outer side then the dsik still has to do some work to fetch different parts. Jul 21, 2010 at 9:56
  • Well I think there will definately be a large amount of attachments. I will try running a compact after the user has removed some of the larger ones. Jul 22, 2010 at 8:04

Similar to what joeqwerty posted, but slightly easier to test: if the root Inbox folder contains a lot of email, try moving most or all of it to subfolders.


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