How many times you had to set up a SMTP service just to test that your .NET application sends the e-mail correctly? I found that I can test the actual application (not the SMTP delivery) by using a bogus SMTP server in the config file.
It pretends to be a SMTP server but it "delivers" the messages by leaving them into a folder you specify.
<system.net>
<mailSettings>
<smtp from="admin@domain.com" deliveryMethod="SpecifiedPickupDirectory">
<specifiedPickupDirectory pickupDirectoryLocation="c:MailFolder"/>
</smtp>
</mailSettings>
</system.net>