thanks @girish and @msbt for chiming in here. I cant see rainloop settings that support a single inbox over multi domains (but i do see how you can setup two or more accounts and simply switch between accounts)
the feature in G SUITE i am aiming to reproduce here, is to have one inbox, that multiple domains are associated with, so emails to any of the domains end up in the single inbox, and upon reply, i can choose which email to reply with. In G Suite you add domains to the domain list, then under the user you can setup an email alias that span across any domain in your domain list. this is why i originly figured this was a feature that maybe cloudron would offer, but i see your point that maybe this is app level feature (just not seeing that feature in either rainloop or sogo, will try nextcloud now)