Microsoft Outlook stores your emails, tasks and appointments as either .pst or .ost files. These two do the same basic job. However, there are some differences that are important, depending on whether you want to back up, restore or move your data.
To explain the difference between PST and OST files, we need to explain some technical history ̵1; specifically the difference between the POP and IMAP e-mail protocol. Here is a quick overview.
What is POP?
The traditional e-mail protocol used for dial-up times was the Post Office Protocol (POP). The current version of POP is version 3 (POP3).
POP downloaded all your emails to the client and deleted them from the email server by default. This meant that only one copy of your emails was on your computer. You can configure POP not to delete e-mail messages from the server.
But at that time, people normally checked their email from only one computer, so it was not necessary to keep a second copy on the server.
When you use POP, all changes you make to your email client are reflected in the email server. If you delete an e-mail message in the e-mail client, nothing happens on the e-mail server and vice versa.
What is IMAP?
Internet Messaging Access Protocol (IMAP) is a modern protocol that downloads a copy of your email from the server to the client on your computer. All changes you make to your email client are synchronized with the server. Deleting an e-mail message on your computer also deletes it from the e-mail server and vice versa.
IMAP fits much more in the modern world. We now have access to the same e-mail account on several devices and most have “always on” broadband or fiber connections and mobile data. Everything you do with your email is synchronized if you use IMAP.
For example, if you send an e-mail from your phone, you can look in the “Sent” folder on your tablet, and the e-mail you sent will also be there. This is why we recommend that you use IMAP unless you have a specific reason to use POP3.
It is also important to note that Microsoft uses the Messaging API (MAPI) rather than IMAP for its email accounts. Although different, MAPI and IMAP synchronize your emails between the client and the email server.
POP, IMAP and PST and OST files
If you have a POP account, Outlook stores all your emails and appointments in a Personal Storage Table (.pst) file. A .pst file can be imported into Outlook. This makes it ideal for moving your email to a new computer or creating a backup that you can save if your computer crashes or becomes inoperable.
Until Microsoft Outlook 2013, PST files were also used for IMAP or MAPI accounts. But as of Outlook 2016, the client stores all your emails and appointments from IMAP and MAPI accounts in an Offline Storage Table (.ost) file.
OST files are automatically synchronized with the e-mail server, as long as you have an Internet connection. Unlike .pst files, however, you cannot import an .ost file into Outlook – because you do not need to.
If you set up Outlook on a new computer and connect to your e-mail account via IMAP or MAPI, all your e-mails will be on the server and they will be downloaded automatically.
RELATED: To set up a POP3 or IMAP account in Microsoft Outlook
The critical difference between PST and OST files is that the contents of a PST are only available in that file. However, the contents of an OST are also available on the e-mail server and other devices that you use to access your e-mail account.
When do you need them?
For the most part, you do not have to worry about PST and OST files. It is unlikely that you will see or directly access either if you are not looking for them.
They are really only important when you are moving data to a new computer or want to archive your emails. If you are using a POP3 email account, you will need to copy the .pst file to your new computer and then import it into Outlook, otherwise you will lose all your emails.
If you use IMAP or MAPI, you only need to be aware of the size of your mailbox on the e-mail server. If you ever reach the maximum size and want to keep all your emails, you need to export a piece of them to a .pst file and then delete them from the email server.
You will still be able to see these emails if you import the .pst file into Outlook, but they will no longer be on the email server.