Running a file outside Grav

I need to run a straight html page outside of Grav that is utilised by a form sending php script that is running in the webroot next to Grav... problem is I can't get the form to find my html page, I assume either because my htaccess is blocking it or Grav is stopping it.

Can I run just a regular html file in the root next to Grav?

So my structure looks like this:

vendor
user
system
logs
images
cache
bin
assets
.htaccess
.gitignore
message
message/submit-error.html
formsend.php

The formsend.php file works, its trying to process my form but it can't find my /message/ dir and the /message/submit-error.html file that I have shown above.

I'd be happy to have the /message/submit-error.html file under users/pages/message/ except that it needs to be able to be processed by the php form so it can add errors etc to the page if there are any, and again, I can't get it to find the file - I assume because its processed by Grav.

How can I get around this?

That should work. In fact I just tested it and it worked fine. Just test by pointing your browser directly to the page:

http://yoursite.com/message/submit-error.html

If that works, then the issue is you need to ensure the correct URL your page is trying reach.