How do I isolate MIAB to a subdomain

I set up MIAB on an ubuntu server which had my website and a client’s website. MIAB seems to have taken over the domain name and my nginx reverse proxy and vhosts are being ignored. I only want MIAB to be operational under the mail.domain.tld such that I can do everything else I want on domain.tld. Is this possible? I don’t have enough money to get a dedicated mail server right now, but I plan to do that in the future.

No. The instructions were very clear.

VPS’s can be had for $2 / month which will run Mail-in-a-Box.

512mb:
https://my.frantech.ca/aff.php?aff=3718&pid=1439

Or if you can splurge for $3.50 / month.
1gb:
https://my.frantech.ca/aff.php?aff=3718&pid=1413