Hi @Fiammenghi Fabrizio ,
By default, the value of the external postmaster address setting is blank. This default value sets the external postmaster address to the value postmaster@<Default accepted domain> for your organization.
You can refer to this link to modify the external postmaster address :
Configure the external postmaster address in Exchange Online | Microsoft Learn
However, there is no built-in feature for setting up postmaster for multiple different accept domains.
Here is a similar thread for you reference :https://learn.microsoft.com/en-us/answers/questions/728499/setting-several-postmaster-adresses-for-each-indiv
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment". Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread