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SMTP Tester

A simple library and GUI client for testing SMTP connections (with or without TLS/SSL), mail delivery, and the .Net system.net/mailSettings configuration section for use with the built-in SmtpClient in .Net.

The goal of SMTP Tester is to answer the question, "Why is my app not sending email?". The intent is to display issues with TCP connections, remote SSL certificates, and negotiating a secure TLS/SSL connection, which aren't usually displayed or surfaced by the SmtpClient in .Net.

To accomplish this, the test client displays details about validation errors with remote SSL certs, but then swallows them and barrels forward to discover what issues might be encountered if the SSL issues are resolved. That way, hopefully most, if not all, of the issues can be revealed with a single test run.

You can compile the project as a WinForms app for the GUI, or change the output to a Class Library to consume the client from other code. You can subscribe to events for log output and errors that occur along the way.

Written in C#.

Features

  • Tests SMTP server connections with or without TLS/SSL.
    • Connects over TCP and sends an EHLO command to enumerate extended SMTP commands on the server.
    • If TLS/SSL is enabled, sends a STARTTLS command to test negotiating a secure connection.
    • Supports TLS 1.0, 1.1, and 1.2. SSLv3 and prior are disabled to mitigate POODLE and other attacks. TLS 1.0 should be disabled to mitigate BEAST attacks, but support in the wild is inconsistent, as TLS 1.1 and above require Windows Server 2008 R2 or OpenSSL 1.0.1 and above.
    • Shows negotiated authentication protocols and X.509 certificate details.
    • Shows SSL certificate validation errors for all certs in the chain, then proceeds with the connection to continue testing.
  • Tests mail delivery by optionally sending a message.
  • Tests the standard .Net-based App.config file settings in the system.net/mailSettings configuration section to verify the default behavior of SmtpClient as it would be used in another application.
  • Logs all output with events, which appear in the GUI or your own app.
  • Fires an event when a response is received to indicate whether the request succeeded or failed. Exceptions are accumulated and thrown at the end of the test within an AggregateException.
  • Uses the Task Parallel Library async/await constructs for efficient, asynchronous I/O without blocking the UI.

Instructions

Configure all SMTP connection settings in the system.net/mailSettings configuration section of the App.config file before building. At runtime, the settings should be configured in the SMTPTester.exe.config file before launching the app. Settings that are configured in the config file are grayed out in the UI.

Testing SMTP Connections

To test an SMTP connection without sending mail, use the Test Connection button.

Testing Mail Delivery

To test sending an email, fill out the available form fields and use the Send Email button.

Consuming the client as a library

The SMTP Tester GUI is essentially a guide on how to consume the SmtpTesterClient class. You can drop the class in your own code, load the app executable as a library, or change the project output to a Class Library and reference it in your own project.

Because a number of facets of the configuration are being tested (and not simply the credentials used for authentication), the test methods don't return a simple success/failure Boolean. Instead, exceptions are thrown when a connection fails. Unless an exception is thrown from within the .Net Framework that stops processing, the connection to the server is made, then the EHLO, STARTTLS, and QUIT commands are sent.

Testing SMTP Connections

To test an SMTP connection without sending mail, call the SmtpTesterClient.TestConnection(...) method.

Testing Mail Delivery

To test sending an email, call the SmtpTesterClient.TestEmail(...) method.

Events
  • Whenever there is log output, a LogOutput event is fired. Subscribe an event handler if you want to display log output in your app.
  • Whenever a response is received from the server, a ResponseReceived event is fired with a Boolean indicating whether it was a success or error code. Subscribe an event handler if you want to know immediately when an SMTP error response is received from the server (generally to change an indicator in the UI). An exception will not be thrown immediately, in order to continue the test. Once the test is complete, if an error was detected in any of the server responses, an AggregateException is thrown that contains all of the server response errors in the InnerExceptions property. If you just want to get all the errors and don't care about showing an immediate indicator in the UI, then don't bother subscribing to this event; just catch the AggregateException and examine the messages of the InnerExceptions to get the error responses.
    • Note: An SMTP response error is defined as a response code that doesn't start with 2 or 3 (essentially in the 200-300 range). Any exceptions thrown by the .Net Framework will be thrown normally, so your try/catch handling should expect to handle both scenarios. You can look for exceptions of type SmtpResponseException to distiniguish SMTP response errors from other exceptions that might also be thrown inside an AggregateException by the .Net Framework.