In case the tenant has multiple instances, when processing Test & Enable Mailbox, sometimes the error notification will be sent to mailbox.
Cannot synchronize outbound mail because mailbox XXXXXXXX is configured to other Microsoft Dynamics 365 organization. It is recommended to overwrite configuration and enable mailbox for primary organization.
Also, please change how to synchronize the organizations other than primary organization.

This message doesn't clearly say which instance is primary organization. The customer cannot know it.
If it is obvious that which instance is primary organization to the mailbox in this error notification, user cannot get confused.