Outlook OST file limit (Classic Outlook for Microsoft 365)

Question

We are planning to migrate from Office 2016 to Microsoft 365 Apps (Outlook Classic).

Could you please confirm:

1. What is the supported OST file size limit for Outlook Classic in Microsoft 365?
2. If we use Exchange Online Plan 2 (100 GB mailbox), does the OST limit still remain 50 GB?
3. What are Microsoft recommended best practices for managing large mailboxes — Online Archive, cache mode settings, etc.?
4. Does Microsoft support increasing the OST size limit beyond 50 GB?

Please share Microsoft official guidance for this scenario.

Answer 1

Dear @Jayathirth Kulkarni,

Good day, and thank you for reaching out to the Q&A forum.

I understand you have several questions regarding the Outlook OST file size limits as part of your migration from Office 2016 to Microsoft 365 Apps (Classic Outlook), and I’m happy to help clarify them. I will address your questions one by one:

1.What is the supported OST file size limit?

The default maximum size for an Outlook OST file is 50 GB across Outlook 2016, 2019, 2021, 2024, and Microsoft 365 Apps (Classic Outlook).

2.Does Exchange Online Plan 2 (100 GB mailbox) change the OST limit?

No. Even with a 100 GB mailbox under Exchange Online Plan 2, the OST file limit remains 50 GB. The mailbox size on the server and the local OST file size limit are independent.

3.Microsoft recommended best practices for large mailboxes

To maintain performance and stability, Microsoft recommends:

• Keeping the OST file size below 50 GB
• Using Online Archive for older emails
• Adjusting the “Mail to keep offline” setting (for example, 3, 6, or 12 months instead of “All”)
• Considering Cached Exchange Mode/Online Mode for very large mailboxes
• Periodically cleaning up large attachments and compacting the OST file

4.Can the OST size limit be increased beyond 50 GB?

Yes, it is technically possible by modifying the Windows Registry. However, Microsoft does not recommend increasing the OST file size beyond 50 GB due to potential risks such as reduced performance, synchronization issues, and a higher likelihood of file corruption.

Some organizations choose to increase the limit (for example, to 75 GB or 100 GB) when necessary, but this should be done carefully and monitored closely.

For references:

Configure Size Limit for PST and OST Files In Outlook | Microsoft Learn

Manage email storage with In-Place/Online Archive mailboxes - Microsoft Support

Exchange Online limits - Service Descriptions | Microsoft Learn

Turn on Cached Exchange Mode - Microsoft Support

I hope this information is helpful. If you have any questions or need further assistance, please feel free to share them in the comments so I can continue to support you. I'm looking forward to your reply.

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Answer 2

1. For Outlook Classic in Microsoft 365, the default maximum size for a Unicode .ost (and .pst) file is approximately 25 GB. In Outlook 2010 and later versions, the maximum size for a .pst file is 50 GB, and the same registry-based mechanism is used to configure limits for both .pst and .ost files. For details on configuring these limits, see 832925 as referenced in the Outlook performance article.
2. Using Exchange Online Plan 2 with a 100 GB mailbox does not change the Outlook client’s data-file behavior. The guidance in the Outlook performance documentation is based on .ost size, not mailbox quota:
   • Up to 5 GB: good experience on most hardware.
   • 5–10 GB: experience becomes hardware-dependent; slower disks may see pauses.
   • 10 GB: short pauses begin to occur on most hardware.
   • Very large (25 GB or larger): increased frequency of short pauses, especially while downloading new messages.
   These thresholds apply regardless of the server-side mailbox size; a very large mailbox does not imply that the local .ost should or will grow to the full mailbox size.
3. Microsoft-recommended practices for large mailboxes and .ost files, based on the provided guidance:
   • Keep .ost size moderate for performance:
     • Aim to keep the active .ost well below 25 GB to reduce application pauses.
     • Be especially cautious once the .ost exceeds 10 GB, as pauses become more common.
   • Use features that reduce local data footprint and sync load:
     • Use Send/Receive groups to manually sync mail when working with very large .ost files (25 GB or larger) to reduce continuous background synchronization load.
   • Reduce mailbox and data-file size:
     • Delete unneeded messages and folders.
     • Compact Outlook Data Files (.pst and .ost) to actually reduce file size on disk. The “Reduce the size of your mailbox and Outlook Data Files (.pst and .ost)” guidance explains that deleting items alone does not automatically shrink the file and that manual compaction is required.
   • For POP vs. Exchange/IMAP/Outlook.com:
     • POP accounts store all data in .pst files.
     • Microsoft 365, Exchange, IMAP, and Outlook.com accounts use .ost files as local copies of the server mailbox. Managing size and compaction of these files is recommended for performance.
4. Microsoft supports increasing the size limits for .pst and .ost files beyond the default via registry configuration, as documented in KB 832925 (“How to configure the size limit for both (.pst) and (.ost) files in Outlook”). However, the performance guidance explicitly warns that as .ost files grow beyond 10 GB—and especially beyond 25 GB—application pauses become more frequent. Any increase beyond the default limits should therefore be weighed against these performance implications and hardware capabilities.

In summary, while registry-based configuration can raise the technical maximum for .ost files, Microsoft’s official guidance emphasizes keeping .ost sizes relatively small (ideally under 10–25 GB) and using mailbox management (deletion, compaction, and controlled synchronization) to maintain good Outlook performance, regardless of the server-side mailbox quota.

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References:

• How to troubleshoot performance issues in Outlook
• Reduce the size of your mailbox and Outlook Data Files (.pst and .ost)
• You may experience application pauses if you have a large Outlook data file