I’d want to split them up so I can let the mails that go to @mailbox.org, be received by a different inbox than my custom domain email. Is this possible?

I’m currently paying for two different email providers, 1 for my custom domain and 1 for the rest (KolabNow). I’d like to reduce my costs and streamline this a bit. Didn’t really know where to post though so I’m trying here.