.NET Code Example for mySMTP with MailKit

Use MailKit to send email from .NET applications through mySMTP. MailKit is the recommended library because Microsoft does not recommend System.Net.Mail.SmtpClient for new development.

Prerequisites

.NET 6 or later
Your mySMTP username and password from the setup email
Your mySMTP hostname from the setup email (shown as relayxx.mysmtp.com in these examples)

Install MailKit

dotnet add package MailKit

Basic send example

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

var host = Environment.GetEnvironmentVariable("MYSMTP_HOST") ?? "relayxx.mysmtp.com";
var user = Environment.GetEnvironmentVariable("MYSMTP_USER");
var pass = Environment.GetEnvironmentVariable("MYSMTP_PASS");

var message = new MimeMessage();
message.From.Add(new MailboxAddress("Your Name", "you@yourdomain.com"));
message.To.Add(new MailboxAddress("Recipient Name", "recipient@example.com"));
message.Subject = "Test email from .NET";
message.Body = new TextPart("plain")
{
    Text = "This is a plain text test message sent through mySMTP."
};

using var client = new SmtpClient();
await client.ConnectAsync(host, 587, SecureSocketOptions.StartTls);
await client.AuthenticateAsync(user, pass);
await client.SendAsync(message);
await client.DisconnectAsync(true);

Console.WriteLine("Message sent successfully");

Replace relayxx.mysmtp.com with your real hostname from the mySMTP setup email. Load credentials from environment variables, never hardcode them in source files.

HTML email with attachment

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

var host = Environment.GetEnvironmentVariable("MYSMTP_HOST") ?? "relayxx.mysmtp.com";
var user = Environment.GetEnvironmentVariable("MYSMTP_USER");
var pass = Environment.GetEnvironmentVariable("MYSMTP_PASS");

var message = new MimeMessage();
message.From.Add(new MailboxAddress("Your Name", "you@yourdomain.com"));
message.To.Add(new MailboxAddress("Recipient Name", "recipient@example.com"));
message.Subject = "HTML email with attachment";

var builder = new BodyBuilder
{
    TextBody = "This is the plain text fallback.",
    HtmlBody = "<h1>Hello</h1><p>This is an HTML message.</p>"
};
builder.Attachments.Add("/path/to/invoice.pdf");
message.Body = builder.ToMessageBody();

using var client = new SmtpClient();
await client.ConnectAsync(host, 587, SecureSocketOptions.StartTls);
await client.AuthenticateAsync(user, pass);
await client.SendAsync(message);
await client.DisconnectAsync(true);

Console.WriteLine("Message sent successfully");

Microsoft recommends against using System.Net.Mail.SmtpClient for new development because it is considered obsolete and does not support modern protocols. Use MailKit instead.

Common errors

535 Authentication failed

Your username or password is incorrect, or your account is not yet activated. Re-enter credentials exactly as shown in the setup email. See Setting up SMTP credentials.

Connection timeout or refused

Your network or host may block port 587. Try port 2525 with SecureSocketOptions.StartTls, or port 465 with SecureSocketOptions.SslOnConnect. See Supported SMTP ports and encryption.

TLS handshake failure

The SecureSocketOptions value does not match the port. Use StartTls for ports 587 and 2525. Use SslOnConnect for port 465.

Test your SPF, DKIM, and DMARC setup with the mySMTP Deliverability Toolbox before sending large volumes.

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.NET Code Example for mySMTP with MailKit

Prerequisites

Install MailKit

Basic send example

HTML email with attachment

Common errors

​Prerequisites

​Install MailKit

​Basic send example

​HTML email with attachment

​Common errors

​Related articles

Prerequisites

Install MailKit

Basic send example

HTML email with attachment

Common errors

Related articles