Support Start > Email > Setting Up An Email Program > Microsoft Outlook > Outlook for Windows File Too Large

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We occasionally hear from customers who see an error message in the Microsoft Outlook mail program for Windows saying that a “message store”, “PST file”, or “OST file” is too large, or has reached its “maximum size”.

If you see this error, keep in mind that Outlook is referring to a file on your own computer. It's not anything to do with our servers. Outlook stores an extra copy of all your mail in a file on your computer, and that file has a size limit (20 GB for Outlook 2007 and earlier, 50 GB for Outlook 2010 and later).

First of all, try Microsoft’s tips to reduce or compact the size of the files.

If that doesn’t help, Microsoft suggests that you archive old mail in Outlook. Archiving moves old messages to a different file on your computer, so it solves the problem of the original file being too large.

If that still doesn't help, or if you don't want to do that, Microsoft explains how technically advanced users can increase the file size limit. Doing that should remove the error, but Microsoft also warns that doing this can make the Outlook program slow or unstable.

A final option if you use IMAP connections is that you can ask us to move mail from your existing mailbox to a new “archive” mailbox on our servers. For example, if your primary address is “address@example.com”, you could ask us to move older mail to a new mailbox called “address-archive@example.com”. You can then view that archived mail using webmail at any time, without adding it to Outlook and having it cause problems there.

This latter option only works with IMAP connections, not POP connections, because if you use POP, moving mail on the server doesn’t automatically remove it from your local computer copy. Also, if you do this, you may need to compact the “.ost” file afterwards, or even remove and re-add the IMAP account in Outlook.

What if none of this fixes it?

Since this is a problem specific to the Outlook for Windows program, Microsoft’s tech support may be able to help.

Or, as an alternative, you may want to consider switching to the free Mozilla Thunderbird mail program, which doesn't have this limitation.