Dedicated Domain Name Required?

1

I have a domain I wanted to set up email for, call it Domain.com, 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 mail.domain.com to the 2nd VPS and keep domain.com and www.domain.com pointed to the original VPS.

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

2

Mail-in-a-Box can also be used as a name server.
so let’s say you have domain.com:
you will point ns1.box.domain.com and ns2.box.domain.com 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 domain.com then.