Send email with C# / .NET over SMTP (MailKit)

dotnet add package MailKit

using MailKit.Net.Smtp;
using MailKit.Security;
using MimeKit;

var host = Environment.GetEnvironmentVariable("SMTP_HOST") ?? "smtp.queensmtp.com";
var port = int.Parse(Environment.GetEnvironmentVariable("SMTP_PORT") ?? "587");
var user = Environment.GetEnvironmentVariable("SMTP_USER");
var pass = Environment.GetEnvironmentVariable("SMTP_PASS");

var message = new MimeMessage();
message.From.Add(new MailboxAddress("Your App", "hello@yourdomain.com"));
message.To.Add(MailboxAddress.Parse("customer@example.com"));
message.Subject = "Your receipt";
message.Body = new BodyBuilder
{
    TextBody = "Thanks for your order #1042",
    HtmlBody = "<h1>Thanks for your order</h1><p>#1042 is confirmed.</p>",
    // Attachments = { "invoice-1042.pdf" }
}.ToMessageBody();

using var client = new SmtpClient();
client.Timeout = 20000;
await client.ConnectAsync(host, port,
    port == 465 ? SecureSocketOptions.SslOnConnect : SecureSocketOptions.StartTls);
await client.AuthenticateAsync(user, pass);
await client.SendAsync(message);
await client.DisconnectAsync(true);

// legacy System.Net.Mail (obsolete in .NET, still common):
// using var smtp = new System.Net.Mail.SmtpClient(host, 587) { EnableSsl = true,
//     Credentials = new System.Net.NetworkCredential(user, pass) };
// smtp.Send("hello@yourdomain.com", "customer@example.com", "Your receipt", "Thanks");