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What is the difference between Outlook PST and OST files?

The Microsoft Outlook logo.

Microsoft Outlook saves your emails, tasks and appointments as either PST or OST files. Both do the same basic job. However, there are a few key differences 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 a little bit of technological history ̵

1; especially the difference between the POP and IMAP email protocols. Here is a brief overview.

What is POP?

The traditional email protocol used during dial-up hours was the Post Office Protocol (POP). The current version of POP is version 3 (POP3).

POP downloaded all of your email to the client and then deleted it from the email server by default. This meant that there was only one copy of your email on your computer. You can configure POP so that e-mail is not deleted from the server.

In those days, people usually only checked their emails from one computer, so there wasn’t really any need to keep a second copy on the server.

If you’re using POP, any changes you make to your email client won’t show up in the email server. If you delete an email in the email client, nothing happens on the email server and vice versa.

What is IMAP?

An email notification on a keyboard.
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Internet Messaging Access Protocol (IMAP) is a more modern protocol that downloads a copy of your email from the server to the client on your computer. Any changes you make to your email client are synchronized with the server. When you delete an email on your computer, it is also deleted on the email server and vice versa.

IMAP is much better suited to the modern world. We are now accessing the same email account on multiple devices and most users have broadband or fiber connections and cellular data. Everything you do with your email is synced when you use IMAP.

For example, when you send an email from your phone, you can look in the Sent folder on your tablet, and the email you sent will also appear there. For this reason, we recommend using IMAP unless you have a specific reason for using POP3.

It’s also important to note that Microsoft Messaging uses API (MAPI) instead of IMAP for its email accounts. Although they are different, MAPI and IMAP synchronize your email between the client and the email server.

POP, IMAP, as well as PST and OST files

The Outlook icon on an Android smartphone.
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If you have a POP account, Outlook stores all of your emails and appointments in a PST (Personal Storage Table) file. A PST file can be imported into Outlook. This makes it ideal for moving your email to a new computer or for a backup that you can save in case your computer crashes or becomes inoperable.

Until Microsoft Outlook 2013, PST files were also used for IMAP or MAPI accounts. As of Outlook 2016, however, the client saves all your emails and appointments from IMAP and MAPI accounts in an offline storage table (.ost).

OST files are automatically synchronized with the email server as long as you have an internet connection. However, unlike PST files, you cannot import an OST file into Outlook as it is not required.

When you set up Outlook on a new computer and connect to your email account using IMAP or MAPI, all of your email is on the server and is downloaded automatically.

CONNECTED: How to set up a POP3 or IMAP account in Microsoft Outlook

The key difference between PST and OST files is that the contents of a PST are only available in that file. However, the content of an OST is also available on the email server and any other device that you use to access your email account.

When do you need them

Most of the time, you don’t have to worry about PST and OST files. They are unlikely to be seen or accessed directly unless you are looking for them.

They really only matter if you want to move data to a new computer or archive your e-mail. 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, all of your emails will be lost.

If you are using IMAP or MAPI, all you need to know is the size of your mailbox on the mail server. If you ever reach the max size and want to keep all of your emails, you need to export part of it to a PST file and then delete it from the email server.

You can then still view these emails when you import the PST file into Outlook, but they are no longer on the email server.

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