You receive the following error during a Copy mailboxes operation:

The destination mailbox is full and can’t receive contents from the source. Clear space in the mailbox to use it as a destination.

When your mailbox size looks to have enough space at the destination, this error usually means that the Recoverable Items folder in the destination mailbox has reached its limit.

Exchange Online mailboxes have a main mailbox quota visible to users and a separate hidden quota used for compliance and recovery.

ShareGate Migrate does not interact directly with the Recoverable Items folder during a Copy mailboxes operation.

However, the app's actions during migration (like creating or updating items) can fill it up, especially when features like Litigation Hold, retention policies, or mailbox audit logging are enabled.

You can use the Exchange Online PowerShell module with the following commands (connected with Connect-ExchangeOnline) to check mailbox limits and usage.

Get information about quotas:

Get current sizes and quota status:

Check if compliance features that may be filling up the Recoverable Items folder are active:

Get information about which of the recoverable item folders are being filled up:

If any are enabled and not required, turning them off can help resolve the issue.

If no hold or policy can be changed, you can follow Microsoft’s guidance to clear the Recoverable Items folder, see their article, Clean up or delete items from the Recoverable Items folder in Exchange Online.