Strictly speaking, you don't need IIS in the mix. You only need IIS (or your own POP/SMTP server) if you actually need management of the mail account you want to interact with at the server level.
You could certainly just write a windows service that uses SmtpClient (built into .NET) to send out emails to your external SMTP server:
http://msdn.microsoft.com/en-us/library/system.net.mail.smtpclient.aspx
And to check emails, you could use a POP client. I've used aspnetpop3 and it's ok, but you can certainly find some other (free) resources for checking POP mailboxes:
http://www.advancedintellect.com/product.aspx?pop3