Writing Unit Test for SMTP Component


I was working on a notification system that allows users to subscribe an email notification based on the status of the database record. An email will be sent to the destination whenever the status is satisfied.

As the system is providing various of email templates for users to choose when they’re defining the subscription. So the challenge to the development team is writing the unit test for each template and ensure the code coverage is more than 90% according to the DevOps strategy.

Writing this post, I would like to share the experience to write the unit tests for SMTP and how to perform the system integration test (SIT) without Exchange Email Server.

I. Writing Unit Tests For SMTP

As in the email template, I allow users to add some predefined parameters into email subject and body as the sample below:

  •  [DateTime]: before delivery email to destinations the system will transform this token to current date time.
  • [RecordId]: the system will pick up the primary key value of the record and replace to this token.
  • [CreatedBy] or [UpdatedBy]: if these tokens are in email subject it will be transformed to account email from active directory. If it is in email body then will be transferred to account name instead.

The requirement of the unit test for this system is able to verify not only template transformation but also the SMTP communication in order to limit the number of the code issues bugs leaking to SIT and UAT.

I found a useful open source library named netDumbster allows to simulate an SMTP service for developers to write the unit tests for SMTP module. This module allows running the unit tests in any build server without any additional installation needed.

Below is the source code for a sample notification service

And here is the unit test for this service.

The configuration of the Unit Test project.

You may face below issue when using this library. Because the SMTP is not stopping property and the unit tests process is still running.

To resolve this issue just simply force stop below processes in the Task Manager and then run the unit tests again.

Here is the result when running the auto build on the VSTS.

I would recommend using [AssemblyInitialize] and [AssemblyCleanup] to start and stop the SMTP emulator instead of [TestInitialize] and [TestCleanup] because start and stop the emulator is taking time and sometimes the SMTP is not stopped properly and keep holding the port 25 so that the next instance is not able to initialize for the second test method.


II. The SMTP Emulator for SIT and UAT.

There are so many reasons that we don’t want The testers using the real Product exchange server to perform the SIT or UAT testing. I don’t want to list out the reason here. However, if you are looking for an alternative solution that allows testers testing the SMTP without exchange server then there is a free and open source tool for that purposes.

The Papercut is an SMTP emulator that saving the incoming emails to a location instead of delivery to the real destinations. This tool provides a WinForms application that allows tester verify the email directly.

There is a Papercut service allows you host the SMTP emulator on to a server in the test environment and share across to all application.

III. Source Code

You can download the demo source code here.