When going over some large files on my computer, I came across quite a few of pst-files and ost-files in various locations.
Some of these files are named after an email address, some are listed multiple times with a (2) or (3) behind it. Others were tagged as an archive.
What are the differences between these pst-files and ost-files?
Which ones should I keep and which ones can I delete?
Understanding the differences between pst-files and ost-files isn’t that hard nor should it be hard to determine whether you’ll still need it or not. However, there are so many little exceptions to this which can make all the difference which makes things hard again.
Below is a short overview of the main differences and the most important exceptions you should know about before you start moving around, rename or delete pst-files or ost-files.
Pst-files vs. ost-files
The main difference between these two file types is that pst-files are being used for “actual” storage and ost-files are being used for “cached” storage.
In the case of the last, the “actual” copy will remain on the mail server and any local changes that you make or new item that you create are also synched to the mail server. The mail server basically holds the “master copy” and the ost-file is “only a local cache”.
The exception here is that Outlook 2010 and previous also used pst-files to cache the contents of IMAP accounts. As of Outlook 2013, IMAP accounts are also cached in ost-files.
Another exception is that some folders within an ost-file do not sync with the mail server. These folder are marked with “This computer only”.
Pst-files vs. pst-archives
Basically, all pst-files are equal; They store their data in the same way and can be reopened on other computers with Outlook.
When a pst-file isn’t directly associated with a mail account (POP3 or IMAP), we refer to them as archives. You can create these on your own and move items to them or use the Archive or AutoArchive feature to automate the moving process based on the date/age property of your email.
Archive pst-files can be created, opened and disconnected via:
- Outlook 2007 and Previous
File-> Data File Management…
- Outlook 2010, Outlook 2013 and Outlook 2016
File-> Account Settings-> Account Settings…-> tab Data Files
Note: When you disconnect a pst-file, you must also close or restart Outlook before the lock on the file will be released.
Pst-files which are associated with a POP3 account can not be disconnected from Outlook without first assigning that account to another pst-file or Exchange mailbox.
Blindingly deleting pst-files is never recommended as they will most likely contain all your mail, calendar appointments, contacts and more. If you are unsure, always take a look into the pst-file first by opening them from within Outlook via:
- File-> Open-> Open Outlook Data File…
There is much more to the pst-file. For more details see: About the Outlook PST-File.
Pst-archives vs. pst-backups
A misconception which sometimes comes up is that the archive.pst is a backup. It definitely isn’t!
Archive.pst is the default name which Archive or AutoArchive uses to move (not copy!) your older items to.
When you don’t have an archive.pst file, don’t worry. By default, AutoArchive is disabled and it only gets created when you enable it. However, you don’t have to use AutoArchive if you don’t want to and living without an archive.pst is perfectly healthy.
Proper pst-backup files are created when you close Outlook and copy the pst-file to a safe backup location. For more on this see: Backup and Restore all Outlook data.
Pst-files named after an email address
Starting with Outlook 2010, new pst-files, which are created when an account is being configured, are named after the email address of the account it is associated with.
In the end, this really is only just a name and not a limitation in any way. When you have multiple POP3 accounts, you can configure them to also deliver to this pst-file even when the email address is different. No problems there!
However, renaming a pst-file is not recommended when it is still associated with an email account. When you do, make sure that you select the correct pst-file when Outlook asks you for the location of the (now missing) pst-file. Selecting a different pst-file can lead to a corrupt mail profile.
Ost-files with “This computer only” folders
As said before, ost-files are “only a cache”. This holds true for Exchange accounts but not always for IMAP accounts in Outlook 2013 and Outlook 2016.
These type of accounts can also have folders within the ost-file which have “This computer only” behind them. In that case, you cannot blindingly delete the ost-file or rename it to .old. Doing so will result in the loss of the content within these folders.
To prevent this see: Don’t risk losing your Contacts and Calendar when using IMAP in Outlook 2013
Opening ost-files or converting ost-files into pst-files
Unlike pst-files, ost-files cannot be opened individually by Outlook. They can only be accessed by Outlook as long as the account which it belongs to is still configured in Outlook.
When you still have the ost-file but the account is no longer configured in Outlook or you moved the ost-file to another computer, we speak of “orphaned ost-files”.
To access the data in such an orphaned ost-file, you’ll need a data recovery tool to convert the ost-file into a pst-file.
There are various tools out there which claim to be able to do this but not all of them are legit. Trustworthy tools which I’ve used before are: