Use a null SMTP server for testing

It’s not uncommon for companies to have SMTP servers locked down so that only a limited set of machines are able to send email. They might do this by requiring authentication or by blocking port 25 for all but “known” server hardware. This makes a great deal of sense in the face of zombies and spam and all manner of email-propagated evil, but when you’re testing an email-enabled application, it can put a real cramp on your progress.

In some cases, you can get away with running a full-blown SMTP / POP server on your desktop, or maybe you’ve got the freedom to set up a machine or a VM. This can be more hassle than it’s worth, though, in some cases. It’s a hassle to set up, it’s going to suck up a noticeable chunk of resources on your desktop, and maybe the biggest issue – you don’t really want to deliver all those emails!

DevNullSmtp console
DevNullSmtp console

Here’s an alternative that works really well.  Use a null SMTP server.  I found one called DevNullSmtp, but there are others available, too.  DevNullSmtp is a 458K Java JAR file, and it requires Java 1.4 or better to run.  In my case, I used a Java HotSpot VM, and I had no problems at all.

Setup is a snap – I created a shortcut to launch the console, and then clicked a button to start the dummy server.  I changed my application’s configuration to use “localhost” as the SMTP server, and I was up and running.

One of the unanticipated benefits of using this server for email is that you don’t have to worry about setting up dummy email addresses for all your receipients because you don’t want them to get test emails.  All the emails simply disappear into this neat little black hole on your desktop.  Of course, you can save all the emails that are sent through this server so you can go back later and check out their contents, and again – this turns out to be a pretty great tool for testing, because you can compare emails generated in a baseline run to emails in a test run to see if they’ve changed.  Not so easy when you’re using real email, right?

This definitely goes on my short-list of development tools – it’s already saved me a ton of time.

Note: I’ve already got a couple of amendments to add.  First, if you want a *really* simple mock server written in C#, you can get one here:  http://code.msdn.microsoft.com/fakesmtpservice

Now, here’s something even more interesting – I found this while unit testing.  I found that if you connect to either of these fake SMTP servers immediately after disconnecting from a previous send, you’ll block on the open TCP/IP socket.  I found that a delay of 100ms or so worked fine to allow the socket to clear so it could accept a connection again.  You can play with that value if you want.  For unit testing, I just put this delay in my [TestCleanup()] routine (the init routine would work, as well).

The code below shows the delay within a single routine – without that pause in the middle, this routine blocks and fails.


[TestMethod()]
public void BasicSMTPTest()
{
SmtpClient client = new SmtpClient("localhost");
using (MailMessage msg = new MailMessage("me@nowhere.com", "you@nowhere.com", "subj", "Test msg"))
{
client.Send(msg);
}
client = null;
// got to be a delay between these requests, or we'll block port 25
Thread.Sleep(100);

SmtpClient client1 = new SmtpClient("localhost");
using (MailMessage msg1 = new MailMessage("me@nowhere.com", "you@nowhere.com", "subj", "Test msg"))
{
client1.Send(msg1);
}
}

[Update: Oct 2, 2009] –  I just read an article on The Accidental Geek that describes two more products that do approximately the same thing:  PaperCut and Neptune.  I haven’t had a chance to try either one of these products yet, but based on Joe’s experience, it may be worth checking them out.

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6 Replies to “Use a null SMTP server for testing”

  1. If someone's testing an email-enabled application I guess they can ask for email access from the technical support team or whoever is in charge. Still, it's good to know there's another way to make it work.
    Sandra @ Cheap VPS

    1. Right. When you’ve got an open enough environment that developers can do that sort of thing, they’ve certainly got more options at their disposal. The fake SMTP server is still sort of nice, though, because you don’t really have to worry about who your “to” addresses are — if you accidentally end up with a support address or a customer address or something like that in an email, the fake SMTP server still lets you see that stuff without any emails actually being sent anywhere — this is a very good thing for testing.

    2. Right. When you’ve got an open enough environment that developers can do that sort of thing, they’ve certainly got more options at their disposal. The fake SMTP server is still sort of nice, though, because you don’t really have to worry about who your “to” addresses are — if you accidentally end up with a support address or a customer address or something like that in an email, the fake SMTP server still lets you see that stuff without any emails actually being sent anywhere — this is a very good thing for testing.

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