Priority over other domain.
Example: I have two or more domains and I need to have one with priority over the other, because this one will have more workflow then others.
Assuming you have this:
Based upon the way iRedMail configured email server I recently installed, both domains are independent of each other so you cannot give one priority over the other. There is a way that you can configure Postfix to alias an email address, but I do not see that option exposed in the iRedMail control panel.
My thoughts are that you either discontinue use of the inferior domain or strongly suggest to your stakeholders to not use it.
Hope that helps.