Safely Testing .NET Code That Does Email Delivery

As a .NET developer you will most likely have come across the need to create and send an SMTP (email) message as part of a solution you've built.  When under development you will have either stubbed out the mail delivery code or will have substituted a test email address for those of the final recipients in a live environment (did your mailbox get full?!).

This approach is simple and works pretty well under development, but you know one day someone will come to you with a production problem relating to mail delivery to multiple recipients with each receiving their own copy of the message.  How do you test this without needing multiple test mailboxes and without spamming the real recipients?

A few years back I learnt of a way to test mail delivery with real email addresses that can be performed locally of a development machine with minimal risk (note I didn't say "no risk") that email will actually be delivered to the intended recipients.  The great thing is you don't need to understand a lot about networking or being a sysadmin god to get this working.

IIS To The Rescue

Yes, IIS.

In this case you don't even need any third party software - just the web application server that most ASP.Net developers know ("and love" is probably pushing the relationship a little though I think).

First off, you will need to install the SMTP feature support for IIS on your local machine.  You can get instructions for IIS 7 from TechNet as well as for IIS 6.  If you're on IIS Express, you're out of luck - it only supports HTTP and HTTPS.

Once you have the SMTP Feature installed you will need to make one important change - set the SMTP server to use a drop folder.  The IIS SMTP process will simply drop files into a location you've selected and there they will sit - each file containing an emaill message.

To make this change open the IIS Manager and select the main server node.  You should see (screenshot below) an option for SMTP E-mail.

Double-click the SMTP E-mail option to open the settings dialog. Notice at the bottom the option labelled Store e-mail in pickup directory - you should select this and then select an appropriate location on disk.

Right, that's the hard bit done.

Run Teh Codez

Now you have a safe place for your test mail to sit you need to ensure that your code is configured to attempt delivery via the SMTP instance you just configured.  You can most likely achieve this by changing your application's settings (they're in an XML config file, right?) so that you use either of "localhost" or "127.0.0.1" as the SMTP host name - you won't need to change the port from the standard port 25.

Now when you run your code you should find that the mail delivery folder you set will be populated with a range of files consisting of a GUID with a .EML extension - each of these is an individual email awaiting your eager eyes.

The files are plain text so can be opened using Notepad or your favourite equivalent - you can view all the SMTP headers as well as the message body.  For extra goodness you can also open these files in Outlook Express (does anyone even use that any more?!) or Outlook as shown below.

I used this approach just recently to help debug some problems for a customer and I could do it using their real email addresses safe in the knowledge that I would not end up spamming them with junk test emails.

Hope you find this approach useful.

Update

I had a reader pass on a service link to Mailtrap which might be more useful if you're looking to test with a large diverse team.