-- Edit:
This advice only valid if you're using .NET
Check this out. If you set it appropriately, it will just store your emails on disk :)
SmtpClient client = ...;
client.PickupDirectoryLocation = @"c:\foo\emails\"; //"
client.DeliveryMethod = SmtpDeliveryMethod.SpecifiedPickupDirectory;
-- Edit
Just in case some people don't get it, this means you don't need another SMTP server for test/dev, you just set the variable appropriately.
-- Edit
For completeness, as marc_s shows below, you can set this in configs nicely via:
<system.net>
<mailSettings>
<smtp deliveryMethod="SpecifiedPickupDirectory">
<specifiedPickupDirectory
pickupDirectoryLocation="c:\foo\emails\" />
</smtp>
</mailSettings>
</system.net>