MailboxExceedsTargetQuotaPermanentException

A cross-tenant mailbox move fails because the source mailbox is larger than the quota the mailbox will have in the target tenant.

Why it happens

The user's mailbox in the source tenant is bigger than the quota in the target tenant. Microsoft lists the same cause and fix for MailboxOverQuotaException, DumpsterOverQuotaException, ArchiveExceedsTargetQuotaPermanentException, MailboxDumpsterExceedsTargetQuotaPermanentException and ArchiveDumpsterExceedsTargetQuotaPermanentException. The dumpster is the Recoverable Items folder, where deleted items are kept.

How to fix it

1. 1

   Step 1

   In the source tenant, connect to Exchange Online PowerShell and enable an archive mailbox for the user if they do not have one.

2. 2

   Step 2

   If the archive is also too big, turn on auto-expanding archiving for the user (or the whole organisation). The main archive must be enabled first. You cannot do this in the Exchange admin center.

3. 3

   Step 3

   Wait while Exchange moves items from the primary mailbox into the archive, and from the archive into auxiliary archive mailboxes. Cross-tenant mailbox migration moves archives and auxiliary archives too.

4. 4

   Step 4

   When the mailbox fits the target quota, migrate the user again.

Enable-Mailbox -Archive user@contoso.com
Enable-Mailbox user@contoso.com -AutoExpandingArchive
# or for the whole organisation:
Set-OrganizationConfig -AutoExpandingArchive

Source

Full guide: How to move mailboxes between Microsoft 365 tenants with cross-tenant mailbox migration →