title: Authentication for website dedicated to specific client?
body: <p>I have a web application / website written in ASP.NET Core. The website is for use by one company and will be accessible over the internet. Because it is for a specific company it therefore has to have authentication for that client only (no public registration).</p>
  
  <p>Is there a service with a C# API that allows me to define what domain is allowed to register and sign in? For example all emails in a specific domain or specific emails in a specific domain e.g. @someCompany.com under google or outlook? I am not yet aware of what email client the customer uses. I am also new to setting up authentication for a web app.</p>
  
  <p><em>This may seem like a '<code>whats the best solution</code>' kind of question but what I am asking for is a solution to a very technical problem. There will be opinions on best solution but I think the question is valid here. Happy to be convinced otherwise.</em></p>