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Exchange Mailbox size and Outlook file sizes

Posted: Thursday, March 5th, 2009 by admin

If you were to look for official recommendations from Microsoft on how big a mailbox or PST file or OST file should be, you likely won’t find any concrete answers.  Just because a file technically CAN be 99GB (just pulled that number out of the air), doesn’t mean it is a good idea to actually have a file that big.

So, these are my own personal recommendations.  Every IT person has their own thoughts on this, so go with what feels right to you.

1.  An Exchange mailbox.  Ideally, I like to see these under 1GB in size.  In fact, under 500MB is even better.  The larger a mailbox is, the more chance there is for corruption, and the longer it takes to restore.  In fact, a lot of repair tools for Exchange mailboxes can’t handle a file much bigger than 1GB. 

2.  An OST file.  An OST is going to be the same size as your Exchange mailbox.  This file is created when you select “Use Cached Exchange Mode” when you set up Outlook to connect to your Exchange mailbox.  What this mode does is download a local copy of everything in your mailbox.  Then, when you’re working in your mailbox, all the changes you make are done locally in your OST file, and in the background that file is syncing with the Exchange Server.  It typically gives a better user experience and everything works faster.  Sometimes, though, these OST files can get corrupt and need to be rebuilt.  The larger the mailbox, the longer this takes, so yet another reason to keep your mailbox size to a reasonable level.

3.  A PST file.  A PST file is a personal folder in Outlook that is stored locally on your computer.  When you archive email, it goes into a PST file.  Most people who want to make their Exchange mailbox smaller will archive their email.  That makes the mailbox smaller, and puts the mail into a PST file.  I recommend that a PST file also not be greater than 1 GB in size.  My favorite way of breaking up mailboxes into archive files is to do it by year.  Create one PST file for each year, and then archive mail for a given year into the corresponding PST file.  There are some people who would have more than 1GB of email for a year, and in that case they might want to break their files up into 6-month chunks…. or possibly even come up with another scheme, such as archiving by customer.


How do you choose to divide up your archives?  How big do you think is reasonable for a mailbox.  Please share!  I’d love to hear other opinions on this.

2 Responses to “Exchange Mailbox size and Outlook file sizes”

  1. zwr says:

    Your PST file is oversized. Microsoft Outlook 2002 and earlier versions limit the size of Personal Folders (PST) file to 2GB. Whenever the PST file reaches or exceeds that limit, you will not be able to open or load it any more, or you cannot add any new data to it. This is called oversized PST file problem. More detailed information can be found at http://www.datanumen.com/aor/problems/2gb-pst.htm

  2. My friend had problem with mails and used next tool-recovery .pst files,as far as I know it is free,utiltity recover data from files in *.ost and *.pst formats,permits to avoid data losses, when mail server administrators face a problem of viruses, hardware damages or software errors,will process a backup copy of your account in *.ost format, that is created automatically by your mail server and convert all messages, contacts and even attachments to a file with *.pst extension, you may open it offline by any email client,start working on corrupted pst file recovery with Outlook pst file recovery software.

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