If you need to develop (or test) on Windows and your program needs to send email, you need an SMTP server.
I just posted this list on Stack Overflow:
There’s also a sendmail.exe which is a very basic system (not actually sendmail, though it accepts basic sendmail arguments.) I’ve used it for things like sending bugzilla notifications and it works with PHP apps. http://glob.com.au/sendmail/
Mercury mail is a more fully featured smtp server that’s fairly simple to configure (though it takes some hunting through it’s weird menus). It’s the mailserver built by Pegasus, and while not exactly “modern” it’s fairly robust. http://www.pmail.com/whatsnew/m32401.htm