We are using the mail receiver for incoming mails.
Let 'say our server domain = domain1
Mail receiver is configured to manage emails sent to xxxxx@domain1.com
Is it possible to configure the same mail receiver to manage as well mails addresses from another domain (domain2), so mails sent to xxxxx@domain2.com are received as well on same discourse server?
Not trivially, although of course since you have full access to the Postfix configuration, you can make it do whatever you like if you fiddle enough. My recommendation is to have the domain2 e-mails handled by a general-purpose MTA that forwards the e-mails to domain1.