Managing Old OST Files during O365 Deployment

Offline Storage Table, also known as Offline Folder or OST file, is an Outlook data file that stores an offline copy of a user’s mailbox data, such as emails, contacts, attachments, calendar items, etc. It provides offline access to the mail items when the network isn’t available or reliable.

When you switch the mail server to Office 365, users need to add and configure their new Office 365 email account in Outlook to continue sending and receiving emails. However, adding an Office 365 account creates a new OST file that stores the mail items synchronized from the Office 365 mailbox. As a result, the old OST file associated with the older profile or email account may enter the orphaned stage.

Since the OST files are encrypted with MAPIEntryID GUID encryption key, one cannot open, access, or import orphaned OST files into any other Outlook client or profile. Instead, it must be converted to Outlook importable PST file format.

However, orphaned OST files may contain mail items that may not have synchronized with the mailbox server. This could result in missing mail items. Thus, you need to manage the old OST files during and after Office 365 deployment to avoid data loss. ?

In this article, we have discussed how to manage the old OST files during Office 365 or Microsoft 365 deployment.

Steps to Manage Old OST Files during Office 365 Deployment

Before switching the email account from your existing mail server to Office 365, you must take a backup of old OST files or ask users to create a local backup of all their mail items in PST format using the Outlook Import/Export wizard. This will help them restore missing mail items as they transition or switch their email account to Office 365 in Outlook.

The steps are as follows:

• Open Outlook and go to File > Open & Export > Import/Export.

• Choose Export to a file and click Next.
• Choose Outlook Data File (.pst) and click Next.

• Select your mail profile or all mail folders and click Next.
• Click Browse to select a location for saving the backup PST file and click Finish.

• Click OK and wait for Outlook to export all the mail items from the OST file to PST format.

However, if you have already deployed and switched to Office 365 or need help in moving OST to Office 365, you can use an OST converter software, such as Stellar Converter for OST. The software can export OST files directly to Office 365 mailbox. In addition, it can export large OST files or mailboxes with complete integrity and precision.

Besides, it converts multiple OST files to corresponding PST files simultaneously via the Batch Conversion option. This can make your job of managing, converting, and moving OST files from local storage to Office 365 easier. You can copy all the OST files of users, convert them using the software, and then manually upload them to Office 365 via the PST Upload method. You may also send or ship the converted PST files to Microsoft in a BitLocker-encrypted drive to upload them to the Azure cloud storage. Once uploaded, you can start the import process.

Tips for Managing Office 365 OST Files

Once all users have moved to Office 365 deployment, create a group policy to keep the OST file size under the recommended limit (~20-25 GB). This is critical to avoid a large OST file that can impact the Outlook performance, cause synchronization issues, and display errors while sending or receiving new mails.

Learn more, How do you manage the large OST file in effective ways??

If your organization uses Virtual Desktop Infrastructure (VDI), it is recommended that you enable and use the Office 365 profile in Online Mode. For this, you may disable the Cached Exchange Mode via Office 365 Group Policy or ask users to disable it while or after setting up the Office 365 profile in Outlook.

Learn more, How to Disable Cached Exchange Mode using GPO?

Conclusion

Migrating or switching to Exchange Online or Microsoft 365 (formerly known as Office 365) requires adequate planning and preparation to avoid downtime and ensure an error-free migration. Also, one of the most critical tasks when deploying Office 365 or Microsoft 365 is migrating or moving the users’ mailboxes. If you are switching from an on-premises Exchange environment, you may follow the cutover or staged migration method based on the Exchange Server version to move the mailboxes.

However, if your organization uses another IMAP-based email server or there are missing mail items from users’ mailboxes after migration, you may need to convert the old OST files into PST and then import them into Office 365. You can use an OST to PST converter tool, such as Stellar Converter for OST to convert OST file to PST or export it directly to Office 365.?