What are .ost and.pst files?
Outlook Data Files (.ost) are used when you have an Exchange account and want to work offline or use or use the default Cached Exchange Mode. This type of data file is also used for accounts that you set up with the Outlook Connector for Outlook.com (formerly Hotmail). Outlook Data Files (.ost) are always copies of items that are saved on a mail server and don’t have to be backed up like Outlook Data Files (.pst).
Source Introduction to Outlook Data Files (.pst and .ost)
My computer seems to lack .pst files.
"4 (of 5) email accounts are @outlook.com, and are listed as 'Exchange ActiveSync', under 'Type'. However, these 4 are all personal accounts on my personal computer. They don't concern any servers or administrators.
1 (of the 5) is my work email, and is listed as 'IMAP/SMTP'."
This confirms that your email is using Microsoft Exchange (in the case of your work IMAP/SMTP account). The
.ost files are from accounts that you have set up with the Outlook Connector for Outlook.com (formerly Hotmail).
Outlook Data Files (.ost) are always copies of items that are saved on a mail server and don’t have to be backed up like Outlook Data Files (.pst).
The adminstrator of your work Exchange Server (in the case of your work IMAP/SMTP account) should be responsible for backing up the server (and your emails).
You might like to confirm with them that backups are actually taking place.
For the other email accounts @outlook.com then Microsoft is the administrator of 'your email server'
Do I need to backup an .ost file?
Some accounts use an offline Outlook Data File (.ost). This is a synchronized copy of the messages saved on a server and that can be accessed from multiple devices and applications such as Outlook. These accounts include IMAP, Microsoft Exchange Server, and Outlook.com accounts.
Offline Outlook Data Files are saved in
It isn’t necessary to back up an offline Outlook Data File (.ost) as it is a copy of the information on the server. If you set up the account again or on another computer or device, a synchronized copy of your messages are downloaded from the server.
Source Create an Outlook Data File (.pst) to save your information
But I really want to backup these 4 .OST accounts
In this case you might like to backup the local
.ost files using your normal backup program (or trust Microsoft not to mess up your accounts)
You can also export your mail as a
.pst file by following the instructions in Exporting and Importing Mailbox Content in Outlook 2013.
.pst file can then also be backed up using your normal backup program.
The advantage of using a
.pst file is you can then import all of your email to another computer running Outlook.
If you access your email account from multiple computers or devices, it’s important to know that when messages are moved to an Outlook Data File (.pst) they are available only on the computer where the file is saved.
Clarifications as per your comments.
You wrote Microsoft Exchange (in the case of your work IMAP/SMTP account). Does this mean a Microsoft Exchange account that also runs on IMAP/SMTP?
Microsoft Exchange can be configured to support IMAP/SMTP. It could also be some other mail server that supports IMAP/SMTP. Only your work IT department can answer this question.
How do I backup .ost files? Do I just copy and paste them onto backup storage (ie an USB, external hard drive)?
You can copy and paste but it is better to use a scheduled windows backup.
It’s important to know that when messages are moved to an Outlook Data File (.pst) : How can email messages be moved to a .pst?
Email messages will be moved if you create a local archive, and drag and drop emails into the local archive.
You can configure Outlook to automatically move your emails to the local
pst file if you wish. See How to Automatically Move Mails From Outlook to PST.
If you export your mail from
.pst you are creating a copy of your email not moving it.