When developing a .Net application that sends out an email it is sometimes useful to write the emails to a local directory rather than send them to a mail server. This is easily done in the SmtpClient configuration in App.Config. The below configuration section will write .eml files to the directory C:\Email (which you will need to create):
<configuration>
<system.net>
<mailSettings>
<smtp deliveryMethod="SpecifiedPickupDirectory">
<specifiedPickupDirectory pickupDirectoryLocation="C:\Email" />
</smtp>
</mailSettings>
</system.net>
</configuration>
Note: if you are using Outlook 2007 you may not be able to open .eml files without an extra step. The file can be opened from the command prompt with the command
"%ProgramFiles%\Microsoft Office\Office12\OUTLOOK.EXE" /eml "<PathToEmlFile>"
or associate the .eml files with a batch file that contains the text
"%ProgramFiles%\Microsoft Office\Office12\OUTLOOK.EXE" /eml %1
A fuller description can be found at http://support.microsoft.com/kb/956693.