public void ConfigureServices(IServiceCollection services)
{
    // Add framework services.
    services.AddMvc();

    //Add MailKit
    services.AddMailKit(optionBuilder =>
    {
        optionBuilder.UseMailKit(new MailKitOptions()
        {
            //get options from sercets.json
            Server = Configuration["Server"],
            Port = Convert.ToInt32(Configuration["Port"]),
            SenderName = Configuration["SenderName"],
            SenderEmail = Configuration["SenderEmail"],
			
            // can be optional with no authentication 
            Account = Configuration["Account"],
            Password = Configuration["Password"],
            // enable ssl or tls
            Security = true
        });
    });
}

public class HomeController : Controller
{
    private readonly IEmailService _EmailService;

    public HomeController(IEmailService emailService)
    {
        _EmailService = emailService;
    }

    public IActionResult Email()
    {
        ViewData["Message"] = "ASP.NET Core mvc send email example";

        _EmailService.Send("xxxx@gmail.com", "ASP.NET Core mvc send email example", "Send from asp.net core mvc action");

        return View();
    }
}