We recently acquired some companies and merged their domains.email/accounts into a single M365 tenant. One acquisition was suppossed to be private. What happened though is a customer emailed the private comapany and a reply from postmaster at otherdomain replied. Now the customer is asking what is up.
Is there a way to change the default postmaster per domain, or no? I see this
Configure the external postmaster address in Exchange Online | Microsoft Docs
but that seems to be only one address.