In Microsoft Office 365 dedicated/ITAR (vNext), a mailbox migration fails, and an error message indicates that the Audit folder is large than 10 GB.
This issue is more common for shared mailboxes but may also occur for a non-shared mailbox that has one or many delegates. In legacy Office 365 dedicated/ITAR, audit mailbox logs are turned on by default. If there are many activities on the mailbox, the logs can fill up fast.
Also, the logs may fill up fast if owner audit logging (which is not turned on by default) was turned to troubleshoot an issue. This owner audit logging should be disabled. For help, see the "More Information" section.
The folder size must be reduced for the migration to succeed. You can clear the items out by setting the value of the AuditLogAgeLimit parameter to 1 day. The mailbox folder assistant will clear out all older audit data, which can take a day or more to process.
Set-mailbox <useridentity> -AuditLogAgeLimit 1.00:00:00 -AuditDelegate $null
As soon as the older content has expired, set the age limit back to the default value of 90 days. Then, retry the migration.
Set-mailbox <useridentity> -AuditLogAgeLimit 90.00:00:00
Delegate auditing captures all user logons and other actions that are taken on a mailbox. This may result in many actions recorded for a heavily used shared mailbox. Therefore, for shared mailboxes, consider disabling delegate auditing and just leaving admin auditing enabled.
Set-Mailbox <useridentity> -AuditDelegate $null