Microsoft Outlook stores your emails, tasks, and appointments as either PST or OST files. These both do the same basic job. However, there are some differences that are important, depending on whether you want to backup, restore, or move your data.
To explain the difference between PST and OST files, we have to explain a bit of technological history—specifically, the difference between the POP and IMAP email protocols. Here’s a quick overview.
What Is POP?
The traditional email protocol used in dial-up times was Post Office Protocol (POP). The current version of POP is version 3 (POP3).
POP downloaded all your emails to the client, and then, by default, deleted them from the email server. This meant only a copy of your emails was on your computer. You could configure POP not to delete emails from the server.
However, in those days, people normally checked their email from only one computer, so there wasn’t really a need to keep a second copy on the server.
When you use POP, any changes you make in your email client aren’t reflected 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?
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 in your email client are synced with the server. If you delete an email on your computer, it’s also deleted on the email server, and vice versa.
IMAP is much more suited to the modern world. We now access the same email account on multiple devices and most people have “always-on” broadband or fiber connections, and mobile data. Everything you do with your email is synced if you use IMAP.
For example, if you send an email from your phone, you can look in the “Sent” folder on your tablet, and the email you sent will be there, as well. This is why we recommend using IMAP unless you have a specific reason to use POP3.
It’s also important to note that Microsoft uses Messaging API (MAPI) rather than IMAP for its email accounts. While they do differ, MAPI and IMAP both sync your emails between the client and 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 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. However, starting with Outlook 2016, the client stores all your emails and appointments from IMAP and MAPI accounts in an Offline Storage Table (.ost) file.
OST files automatically synchronize with the email server, as long as you have an internet connection. Unlike PST files, though, you can’t import an OST file into Outlook—because you don’t have to.
If you’re setting up Outlook on a new machine and connecting to your email account via IMAP or MAPI, all of your emails will be on the server, and they’ll download automatically.
The critical difference between PST and OST files is the contents of a PST are only available in that file. The contents of an OST, however, are also available on the email server and any other device you use to access your email account.
When Do You Need Them?
Most of the time, you won’t have to worry about PST and OST files. You’re unlikely to see or directly access either unless you go looking for them.
They’re really only important when you’re moving data to a new computer or want to archive your emails. If you use a POP3 email account, you’ll have to copy the PST file to your new computer, and then import it into Outlook, or you’ll lose all your emails.
If you use IMAP or MAPI, you only need to be aware of the size of your mailbox on the email server. If you ever reach the maximum size and want to keep all of your emails, you’ll have to export a chunk of them into a PST file, and then delete them from the email server.
You’ll then still be able to view those emails if you import the PST file into Outlook, but they’ll no longer be on the email server.
This was originally posted by HowtoGeek.