Not only item count and mailbox size collected in a Collect Mailbox Statistics task, but also other attributes. For example: LastLogonTime MaxSendSize MaxReceiveSize
原因
Mailbox had setting for MaxSendSize and MaxReceiveSize set to -1 / unlimited in the source tenant. In cloud Exchange environment Unlimited is invalid value.
Get the current setting: get-mailbox <mbx> | FL
対策
Adjust the incorrect value in Exchange admin center and run Collect Mailbox Statistics again.
