I'm troubleshooting a Microsoft 365 user whose 100GB mailbox is already about 80GB full. Outlook Classic reaches the practical 40–50GB OST limit when cached mode is set to download everything, but online mode feels too slow for the user. We can adjust the download period, but a company GPO forces the cache setting to "All," and changing that policy currently isn't approved. The user has years of historical mail, wants to retain it, and deleting tens of gigabytes would be difficult. We also don't currently have permission to use an archiving solution. Given those constraints, are there any reliable ways to provide a better Outlook Classic experience, or is management going to have to choose between changing the GPO, enabling archiving, and accepting reduced performance?
4 Answers
This is partly a policy and retention issue rather than a technical one. Management needs to decide whether the organization will fund archiving, change the caching GPO, or enforce a retention period. Keeping every message indefinitely “just in case” creates storage, compliance, discovery, and performance problems. Emptying Deleted Items and reviewing Sent Items may help a little, but it won’t solve an 80GB mailbox by itself.
The proper fix is Exchange Online Archive. It keeps older messages available in Outlook while moving them out of the primary mailbox, and it can scale far beyond the local OST limit. A retention policy can automatically move mail older than a chosen age, such as two years, while preserving the folder structure. Once the primary mailbox is reduced to a reasonable size, cached Outlook should be much more stable.
It’s often easiest to describe it as a PST stored in the cloud: the user can still search and open historical mail, but Outlook doesn’t have to cache the entire mailbox locally.
If the GPO cannot be changed, the practical choices are online mode, a supported archive, or a different mail client with more flexible offline-sync controls. New Outlook may be worth testing, and some alternative clients can limit what is downloaded, but they may not provide the same calendar, contacts, shared-mailbox, and GAL functionality as Outlook Classic. Moving mail to a shared mailbox and disabling its local caching is another possible design, but it needs careful permission, licensing, search, and compliance review.
There really isn’t a reliable Outlook Classic workaround for caching 80GB. Increasing the OST size through registry or policy settings may appear to work, but large OST files are prone to slow searches, synchronization problems, corruption, and crashes. That should only be considered a temporary emergency measure, not a supported solution.
If a local archive is approved instead, split it by year and keep the PST files backed up somewhere that isn’t OneDrive. PST files should not be stored in a synchronized folder because synchronization conflicts can damage them.
Retention rules should be agreed on with legal or compliance rather than improvised by IT. Important records can be preserved while ordinary correspondence is automatically removed or archived.