When Outlook is connected to a Microsoft Exchange or Microsoft 365 account, it can keep a local copy of mailbox information in an OST file. This allows users to work with their emails even when they temporarily lose internet access.
The OST file works as a locally cached copy of mailbox data, while the original mailbox remains on the server. Outlook regularly synchronizes changes between the two.
How Does OST Synchronization Work?
When a new email arrives, it is stored in the server-side mailbox. Outlook then synchronizes the information and updates the local OST data.
The same process works when users make changes in Outlook. Actions such as sending an email, deleting a message, moving an email to another folder, or marking a message as read are synchronized with the server when a connection is available.
This keeps the Outlook mailbox and server mailbox consistent.
What Happens When Outlook Is Offline?
One of the main benefits of Cached Exchange Mode is that users can continue working with locally available mailbox information when they are offline.
For example, users may be able to read previously synchronized emails or compose new messages. Changes that require server communication are synchronized after Outlook reconnects.
The amount of email stored locally can also depend on Outlook's offline mail settings. Therefore, an OST file may not always contain the entire mailbox history.
Are Attachments and Folders Synchronized?
Yes. Synchronization can include email folders and attachments along with the messages.
For example, if an email containing a PDF or spreadsheet is available in the locally synchronized mailbox, the attachment can also be accessed through Outlook.
This is why checking only the number of emails is not enough when verifying mailbox data. Important folders, attachments, recent messages, and older emails should also be checked.
Why Can OST Synchronization Take Time?
Synchronization speed can depend on several factors, including:
Mailbox size
Number of emails and folders
Internet connection
Attachment sizes
Offline storage settings
Computer performance
Large mailboxes may take longer to synchronize because Outlook has more information to process.
What Happens When an OST File Is No Longer Connected?
An OST file is associated with the Outlook account and profile that created it. If an account is removed, a profile changes, or an old mailbox needs to be accessed separately, the OST data may become difficult to use through normal Outlook functions.
In such cases, converting the available data into another format may be useful. PCDOTS OST Converter can be considered when OST data needs to be converted for migration or further access.
Final Thoughts
OST synchronization allows Outlook to maintain a local working copy of Exchange or Microsoft 365 mailbox data. New messages, folder changes, and other mailbox updates can be synchronized between Outlook and the server.
Understanding this process is useful when troubleshooting synchronization issues, managing large OST files, or preparing Outlook data for migration.
