.NET (ASP.NET Core)

using System.Net.Http.Headers;
using System.Text.Json;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Filters;

public class DisposableEmailFilter : IAsyncActionFilter
{
    private readonly IHttpClientFactory _httpFactory;
    private readonly IConfiguration _config;

    public DisposableEmailFilter(IHttpClientFactory httpFactory, IConfiguration config)
    {
        _httpFactory = httpFactory;
        _config = config;
    }

    public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next)
    {
        var email = context.ActionArguments.Values
            .OfType<string>().FirstOrDefault(v => v.Contains("@"));

        if (email != null)
        {
            try
            {
                var client = _httpFactory.CreateClient();
                client.DefaultRequestHeaders.Authorization =
                    new AuthenticationHeaderValue("Bearer", _config["DG_KEY"]);

                var response = await client.GetAsync(
                    $"https://api.disposableguard.com/v1/check?email={Uri.EscapeDataString(email)}");

                if (response.IsSuccessStatusCode)
                {
                    var json = await response.Content.ReadAsStringAsync();
                    var data = JsonDocument.Parse(json).RootElement;
                    if (data.GetProperty("is_disposable").GetBoolean())
                    {
                        context.Result = new BadRequestObjectResult(
                            new { error = "Please use a real email address." });
                        return;
                    }
                }
            }
            catch { /* fail-open */ }
        }

        await next();
    }
}