having multiple postmaster domains

Hello,

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

https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/configure-external-postmaster-address

but that seems to be only one address.

thx