Debug HTTP is easy, you have all sort of tools to do it (like Fiddler). What about SMTP?
How to Debug SMTP Communications?
My target system is Windows.
Suggested tools:
Try Ethereal - its a free network protocol analyzer.
The SMTP protocol is all ascii, so once you see whats inside the TCP connection, you should be good to go.
It will take a bit of work learning how to use Ethereal.