How to Fix Exchange 2016 Hybrid Mail Queue File Growing Beyond 90 GB
Question details
The on-premises Exchange 2016 mail.que file continues to grow uncontrollably over 90 GB even after all users have been migrated to Exchange Online.
- Product
- Microsoft Exchange 2016
- Device & OS
- not provided
- Scenario
- Managing an on-premises Exchange server in a hybrid environment where all mailboxes have already been migrated to Office 365.
- Observed behavior
- The mail.que file expands beyond 90 GB. Deleting the file only provides temporary relief, as it is immediately recreated and resumes its rapid growth.
Avoid repeatedly deleting the mail.que file before identifying the root cause, as doing so destroys vital diagnostic information and can result in the loss of in-transit messages.
Contact Microsoft Business Support for Advanced Diagnostics
Engage directly with Microsoft support engineers to deeply analyze your hybrid deployment's transport data and server health.
In a complex Exchange Hybrid environment, unchecked growth of the transport database often points to underlying misconfigurations in connectors or message tracking.
A Microsoft engineer can utilize dedicated diagnostic tools to isolate the exact mail flow loops or stuck queues without compromising your server's data integrity.
Navigate to the Microsoft Global Customer Support portal via the official support website.
Choose your region and country to obtain the appropriate dedicated phone number for Business Support.
Before calling, document your current Exchange 2016 build, Office 365 subscription details, and the timeline of your hybrid migration.
Work with the support engineer to review your message-tracking logs, mail flow, and receive connectors to determine why transport data is accumulating.
Review Transport Queues and Connectors Manually
Check your local Exchange environment for stuck messages or misconfigured mail flow connectors that could be causing data to stockpile.
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Frequently Asked Questions
Why does the mail.que file grow so rapidly in a hybrid setup?
The mail.que file acts as a temporary storage database for messages in transit. It grows rapidly if messages are stuck due to routing failures, if aggressive message tracking is enabled, or if misconfigured connectors create mail loops between on-premises Exchange and Office 365.
Is it safe to delete the mail.que file to free up disk space?
No. Deleting the mail.que file can lead to permanent loss of pending emails and destroys critical diagnostic data that support engineers need to resolve the underlying issue. It should only be done as a last resort under Microsoft Support's guidance.
How can I check if messages are stuck in my Exchange queue?
You can check for stuck messages by opening the Exchange Management Shell and running the 'Get-Queue' command. This will display all active queues and highlight any that have a massive backlog or are in a 'Suspended' state.
