I would check out hMailServer. It not only add's SMTP support, but it also if wanted can support IMAP and POP3 protocols too.
hMailServer is a free, open source, e-mail server for Microsoft
Windows. It's used by Internet service providers, companies,
governments, schools and enthusiasts in all parts of the world.
It supports the common e-mail protocols (IMAP, SMTP and POP3) and can
easily be integrated with many existing web mail systems. It has
flexible score-based spam protection and can attach to your virus
scanner to scan all incoming and outgoing email.
There is also mailEnable Standard Edition too,
Standard Edition - Unlimited Domains/Users MailEnable Standard Edition
provides robust SMTP, IMAP, POP3 and Webmail services for Microsoft
Windows servers. Simple to install, with powerful administration
software means that your mail server will be up and running quickly.
This edition is free, contains no spyware or adware, for both personal
and commercial usage, with no time or user restrictions.