Hi,
I checked your website and it seems that you configured your checkout to use the "guest" mode only.
In that case, you can use an email address as much as you want to place orders on the website.
The only requirement is that you don't already have a user account with that email addres on your website.
I suppose that you must have tested it with one of your email addresses that you actually used for a user account of yours to access the backend but normal users of your website wouldn't have that issue as they don't create user accounts and only do guest checkouts.