With an http request, you can test if a user’s Exchange credentials are valid.

Here is an example in c#

string username = "...@...";
string password = "...";
string ewsEndPoint = "https://.../EWS/Exchange.asmx";

var byteArray = Encoding.ASCII.GetBytes(username + ":" + password);

var authenticationHeaderValue = new AuthenticationHeaderValue("Basic", Convert.ToBase64String(byteArray));

HttpClient client = new HttpClient();
client.DefaultRequestHeaders.Authorization = authenticationHeaderValue;

var httpResponseMessage = await client.GetAsync(ewsEndPoint);

It’s a simple way to use your Exchange as an authentication provider.