MissingExchangeGuidException

A cross-tenant mailbox move fails because the target MailUser does not have the ExchangeGuid of the source mailbox.

Why it happens

For a cross-tenant move, the MailUser (a mail-enabled user with no mailbox) in the target tenant must carry the same ExchangeGuid as the source mailbox. The target MailUser is missing the correct value, so the move cannot proceed.

How to fix it

1. 1

   Step 1

   In the source tenant, connect to Exchange Online PowerShell and read the ExchangeGuid of the source mailbox.

2. 2

   Step 2

   In the target tenant, stamp that value on the MailUser with Set-MailUser.

3. 3

   Step 3

   Resume the migration batch.

Get-Mailbox user@contoso.com | Format-List ExchangeGuid   # source tenant
Set-MailUser user@fabrikam.onmicrosoft.com -ExchangeGuid 4e3188c6-39f5-4387-adc7-b355b6b852c8   # target tenant

Source

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