Request for Solution – Gmail Data Exceeding 80GB Causing Outlook PST Issues

Question

Dear Team,
One of our Gmail users has exceeded 80 GB of mailbox data. We are currently accessing the mailbox through Outlook, and we are facing storage and PST corruption issues.

When the mailbox size approaches 50 GB, the Outlook PST file becomes corrupted. For other users, we usually take a backup before reaching 50 GB, but in this case the mailbox has already exceeded that limit.

Additionally, we are unable to take a proper backup directly from Google. We also tried creating a new Outlook profile and syncing the mailbox in parts (first half and second half), but the backup process is still not working.

Since we are currently working only through Outlook, kindly suggest the best possible solution to safely back up or manage this large mailbox.

Answer 1

Hi @Airlock Team

As my research, the safest and most practical approach to back up and manage this large Gmail mailbox while continuing to use Outlook is to avoid using a single large PST file. I would recommend splitting the mailbox into multiple smaller PST files, keeping each Outlook data file well below the 50 GB technical limit as AI's suggestion. For stability and to minimize the risk of PST corruption, each PST should ideally be maintained at approximately 20 to 25 GB or less. 

You could consider exporting emails to PST files in Outlook by year for backup purposes. For further information, you can refer via: 

How to export emails by date range to Excel file or PST file in Outlook? 

Note: Microsoft is providing this information as a convenience to you. These sites are not controlled by Microsoft, and Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please ensure that you fully understand the risks before using any suggestions from the above link. 

The Outlook application should be used primarily for active and recent emails only, rather than as a long‑term archive or backup solution. Older emails should be moved into separate, smaller PST files for archival purposes, ensuring Outlook performance and data integrity remain stable. This approach aligns with best practices for Outlook data management and significantly reduces the risk of storage issues and PST corruption when handling very large mailboxes. 

I hope this helps. 

If you have any additional concerns, feel free to comment below. I would be more than happy to assist. 

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Answer 2

For an 80‑GB Gmail mailbox accessed only via Outlook, the key is to avoid hitting Outlook’s data‑file limits and performance thresholds while still getting a usable backup.

Use these principles from the Outlook data‑file guidance:

1. Keep individual Outlook data files well below 50 GB
   • Outlook 2010 and later support a maximum of 50 GB per .pst by default. Larger files are more prone to corruption and performance issues.
   • Performance guidance indicates that >10 GB data files start to cause pauses, and very large (25 GB or larger) files significantly increase pauses and issues during normal operations.
2. Split the mailbox into multiple smaller PSTs instead of one huge file
   Since the mailbox is ~80 GB, create several PSTs (for example, 10–20 GB each) and move/archive data into them from Outlook:
   • Create multiple local PST files (e.g., Gmail_2018-2019.pst, Gmail_2020-2021.pst, etc.).
   • Move or archive older mail (by date, project, or folder) into these PSTs in batches, keeping each PST under ~20–25 GB to stay within the “good/acceptable” performance range described for Outlook data files.
   • This reduces the risk of corruption because no single PST approaches the 50‑GB technical limit.
3. Compact PSTs after moving data
   Deleting or moving items out of a PST does not automatically shrink the file. After moving large volumes of mail to other PSTs, compact each PST to reduce its physical size and improve stability:
   • Use Outlook’s manual compact feature for Outlook Data Files (.pst).
   • Compacting helps reclaim white space and can reduce the chance of hitting the size thresholds that trigger issues.
4. Reduce OST size if using Cached Exchange Mode with Gmail (IMAP)
   If Outlook is using an .ost (cached mode) for the Gmail account:
   • Large .ost files (especially >10 GB, and 25 GB or larger) cause application pauses and instability.
   • Use mailbox cleanup and archiving to move older items out of the cached mailbox into local PSTs, reducing .ost size.
   • For Outlook 2013 and later, use features like Sync Slider or synchronization filters (if applicable) to limit how much history is cached locally, keeping the .ost smaller.
5. When corruption has already occurred
   • If a PST is already corrupted near 50 GB, repair it and then immediately split it into smaller PSTs and compact them.
   • Going forward, avoid letting any single PST grow beyond ~25 GB to stay within the recommended performance envelope.
6. Plan an ongoing archive strategy
   For this user and others with very large mailboxes:
   • Implement a policy to regularly move older mail from the live Gmail/IMAP store into multiple local PST archives.
   • Ensure each archive PST remains under the recommended size thresholds to avoid the recurring corruption and performance issues.

This approach uses Outlook’s supported limits and behavior: multiple smaller PSTs, compacted regularly, instead of a single very large file, is the safest way to back up and manage an 80‑GB mailbox when working only through Outlook.

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References:

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