Microsoft 365 Exchange Backup Failure

Product version: 10.4

Last Updated: 11 October 2021


After running a Microsoft 365 Exchange backup job multiple times, the job fails with the following error:

Action failed. Backup of the <Mailbox name> mailbox has failed.

In addition, the respective SaaS repository becomes inaccessible with the following errors:

The <Repository name> backup repository is inaccessible.

The <Transporter name> transporter assigned to the <Repository> backup repository is inaccessible. Make sure the transporter is available.


Despite one of the errors reporting that the assigned transporter is unavailable, the Transporters tab reveals no such error. The issue is related to the OS and/or hardware used for the assigned transporter; with default settings, the system lacks the resources to create a new Java thread necessary for the transporter.


To solve this issue, try the following:

  1. Enable Expert mode in your instance by adding “expert” (or “&expert”) to the end of the URL of the Settings page.

  2. Click the Expert tab and scroll down to find “system.transporter.modern.thread.pool.size".

  3. Reduce the value of this parameter from 1000 to 200.

  4. Restart the C++ transporter (bhsvc service)—this will automatically restart the Java transporter with the new settings.

  5. Re-test the issue.