If you are developing in ASP.net using the built-in mail libraries, a lesser-known configuration setting is to use the following:
<configuration>
. . . more config stuff here . . .
<system.net>
<mailSettings>
<smtp deliveryMethod="SpecifiedPickupDirectory" from="noreply@testdomain.org">
<specifiedPickupDirectory pickupDirectoryLocation="c:\smtp\"/>
</smtp>
</mailSettings>
</system.net>
</configuration>
All your application generated emails will be dumped into this directory during development. No smtp server needed!