>I am planning to write spam detection s/w. Are there any rules or conventions for what constitutes a valid email address after the '@'? For example, .com is valid, .com.it is valid--are there any general rules? Thanks

Check http://regexlib.com/. They've a buch of regular expressions for email validation. You can find RFC822: Standard for ARPA Internet Text Messages at http://www.w3.org/Protocols/rfc822/Overview.html