Dedicated Domain Name Required?

I have a domain I wanted to set up email for, call it, I already have a Wordpress site hosted on a DigitalOcean Droplet for this domain. I set up a new, seperate Droplet for Mail-in-a-Box, but it seems that I have to also point my root domain to the mail server for this to work. I was hoping to point to the 2nd VPS and keep and pointed to the original VPS.

If I need to get a cheap throw away domain (say, to act as the main domain for the mail server, then set up later for email, I can do that.

Mail-in-a-Box can also be used as a name server.
so let’s say you have
you will point and to your Mail-in-a-Box. Then you can use the admin interface to set up domain records, which could also point to your already owned Wordpress site. I am pretty sure your wordpress site could still use then.