{"id":425,"date":"2026-09-21T12:55:00","date_gmt":"2026-09-21T18:25:00","guid":{"rendered":"https:\/\/photonconsole.com\/blog\/?p=425"},"modified":"2026-09-21T07:24:46","modified_gmt":"2026-09-21T12:54:46","slug":"sending-email-in-net-and-c-smtpclient-mailkit-and-production-setup","status":"publish","type":"post","link":"https:\/\/photonconsole.com\/blog\/sending-email-in-net-and-c-smtpclient-mailkit-and-production-setup\/","title":{"rendered":"Sending Email in .NET and C#: SmtpClient, MailKit and Production Setup"},"content":{"rendered":"\n

Most C# email tutorials start with System.Net.Mail.SmtpClient<\/code>. It is built into the framework, the code is short, and it works on the first test. Then you point it at a production mail server on port 465 and the connection hangs until it times out. Or it works for a week and starts throwing under load.<\/p>\n\n\n\n

Microsoft’s own documentation now recommends against using that class for new development. This guide explains why, shows the modern replacement, and covers the production pieces most examples skip: configuration, async sending, background queuing in ASP.NET Core, and the port setting that catches almost everyone.<\/p>\n\n\n\n

Quick Answer: How Do You Send Email in C#?<\/h2>\n\n\n\n

Use MailKit with an authenticated SMTP relay:<\/p>\n\n\n\n

dotnet add package MailKit<\/code><\/pre>\n\n\n\n
using MailKit.Net.Smtp;\nusing MailKit.Security;\nusing MimeKit;\n\nvar message = new MimeMessage();\nmessage.From.Add(new MailboxAddress(\"Your App\", \"noreply@yourdomain.com\"));\nmessage.To.Add(MailboxAddress.Parse(\"user@example.com\"));\nmessage.Subject = \"Your verification code\";\nmessage.Body = new TextPart(\"plain\") { Text = \"Your code is 492019.\" };\n\nusing var client = new SmtpClient();\nawait client.ConnectAsync(\"smtp.photonrelay.com\", 587, SecureSocketOptions.StartTls);\nawait client.AuthenticateAsync(smtpUser, smtpPass);\nawait client.SendAsync(message);\nawait client.DisconnectAsync(true);<\/code><\/pre>\n\n\n\n
The library handles the protocol correctly. What decides delivery is the server you authenticate against and whether your domain is authorised to send. An authenticated relay such as PhotonConsole<\/a> covers that side.<\/p>\n\n\n\n
Why Not System.Net.Mail.SmtpClient?<\/h2>\n\n\n\n
The Microsoft documentation for SmtpClient<\/a> states that it is not recommended for new development because it does not support many modern protocols, and points developers to MailKit instead. The class still exists and still compiles, which is why so many tutorials continue to use it.<\/p>\n\n\n\n
Capability<\/th>System.Net.Mail.SmtpClient<\/th>MailKit<\/th><\/tr><\/thead>
STARTTLS on port 587<\/td>Yes<\/td>Yes<\/td><\/tr>
Implicit SSL on port 465<\/td>No<\/td>Yes<\/td><\/tr>
True async API<\/td>Limited<\/td>Yes<\/td><\/tr>
Modern authentication methods<\/td>Limited<\/td>Yes<\/td><\/tr>
Detailed protocol errors<\/td>Minimal<\/td>Yes<\/td><\/tr>
Recommended for new code<\/td>No<\/td>Yes<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
The second row causes the most production failures, covered next.<\/p>\n\n\n\n
Why .NET Email Fails in Production<\/h2>\n\n\n\n
\"Comparison
The built-in SmtpClient cannot do implicit SSL, which is why port 465 hangs.<\/figcaption><\/figure>\n\n\n\n
Most failures come from configuration or authentication rather than faults in the framework. These five account for the majority.<\/p>\n\n\n\n
1. Port 465 With System.Net.Mail<\/h3>\n\n\n\n
The built-in SmtpClient<\/code> only supports STARTTLS, where the connection starts plain and upgrades. Port 465 expects encryption from the first byte. Setting EnableSsl = true<\/code> with port 465 does not produce implicit SSL \u2014 it produces a connection that waits for a plaintext greeting that never arrives, then times out.<\/p>\n\n\n\n
2. Credentials Committed in appsettings.json<\/h3>\n\n\n\n
SMTP passwords placed in appsettings.json<\/code> end up in source control and in every build artefact. Use user secrets in development and environment variables or a secret store in production.<\/p>\n\n\n\n
3. Synchronous Sends Inside Request Handlers<\/h3>\n\n\n\n
A blocking send inside a controller ties up a thread for the entire SMTP conversation. Under load this exhausts the thread pool and slows every request on the server, not only the ones sending mail.<\/p>\n\n\n\n
4. No Authentication Records on the Sending Domain<\/h3>\n\n\n\n
Without SPF and DKIM, receiving servers cannot verify your application is authorised to send as your domain, in line with Google’s sender guidelines<\/a>. Our guide to SPF, DKIM and DMARC<\/a> covers the records, and the free email deliverability checker<\/a> shows what your domain currently publishes.<\/p>\n\n\n\n
5. Outbound SMTP Blocked by the Host<\/h3>\n\n\n\n
AWS, Google Cloud and Azure restrict outbound port 25 by default. AWS documents its port 25 throttle removal process<\/a>, though removal does not solve the reputation problem underneath. Our breakdown of SMTP connection timeouts<\/a> covers the diagnosis.<\/p>\n\n\n\n
Common Mistake<\/p>\n\n\n\n
Using System.Net.Mail.SmtpClient<\/code> with port 465 and EnableSsl = true<\/code>, then spending hours checking credentials. The credentials are fine. That class cannot do implicit SSL, so the connection hangs before authentication is ever attempted. Switch to port 587, or move to MailKit with SecureSocketOptions.SslOnConnect<\/code> if the provider requires 465.<\/p>\n\n\n\n
Quick Fix<\/p>\n\n\n\n
SMTP Send Hangs or Times Out<\/p>\n\n\n\n