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OST file

(also offline storage table file)

OST file definition

An OST file is a digital file that stores a copy of your email messages, contacts, and calendar items from an email account (like Microsoft Exchange) on your device. Because it saves this copy on your computer or phone, you can read and compose emails even when you're not connected to the internet. When you’re back online, the email client syncs the changes you made locally in the OST file.

See also: business email compromise

How OST files work

  • When you set up an email account in an email client like Microsoft Outlook, it creates an OST file on your computer.
  • As you receive emails, your email client downloads copies of these emails from the email server and stores them in the OST file on your computer.
  • The OST file acts as a local storage of your email messages, folders, contacts, and calendar items. Thanks to the OST file, you can access it even when you don’t have a stable internet connection.
  • When you connect to the internet, your email client synchronizes the changes you made locally in the OST file (like sending new emails or moving messages) with the email server.
  • The OST file serves as a backup of your email data. If there are server issues or data loss, you can rely on the data stored in the OST file to recover your emails and other information.
  • OST files are typically encrypted and secured to protect your email data from unauthorized access.