Outlook 2010 is also out of mainstream support, so hopefully if you are still running Outlook 2010 you are planning an upgrade in the near future. Hopefully you’re not running Outlook 2007 or 2003, both of which are unsupported today. Outlook 20 and earlier support an OST file size of up to twenty gigabytes (20GB).Outlook 2010 and later support an OST file size of up to fifty gigabytes (50GB).

The OST file format has a maximum file size that depends on the version of Outlook you’re running: OST files are Outlook offline data files. Outlook’s local cache of mailbox data is stored in a file format known as an OST file. When they reconnect to a network, their emails are sent, and any new emails are downloaded for them to read. Users can read and compose emails without network connectivity, such as when traveling. Second, Outlook cached mode allows users to work with mailbox data while offline. They use Outlook over slow networks such as home internet connections and public Wi-Fi. Users connect to their mailbox over longer distances.

These days, Exchange mailboxes are hosted on servers on centralized datacenters, or in the cloud in Office 365.

In the old days, when Outlook was primarily used to connect to Exchange servers that were close to the end user, on-line mode was good enough. Outlook performance in “on-line mode” is not all that great. When you connect Outlook to an Exchange account, Outlook downloads a copy of your mailbox content to a local cache.

How to Manage OST File Sizes Using the Registry.

How to Manage OST File Sizes Using the Outlook Cached Mode Slider.

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