To amplify davetu 's comments. When you install Mail in a Box, you can then login to an admin website (served by the box). There you will find a page named ‘External DNS’. On that page will be all of the details you need to copy in to your existing DNS provider’s page.
If you do not want to host your website on MIAB then leave that setting unchanged.
The main thing you need to change/add is an A record pointing to the IP address of your MIAB. Like this: A box.example.com 22.214.171.124 (from your MIAB host/virtual private server)
Then an MX record pointing to the address of your MIAB. Like this: MX box.example.com
Do follow the set up guide step by step but ignore the instructions about ‘glue records’ or NS records as you don’t need that if your DNS is external.
Hope this helps.