What exactly do you mean when you talk about "Spam"?
When used in the context of e-mail, Spam is another word for UBE. UBE stands for "Unsolicited Bulk E-mail".
The opposite of Spam is ["Ham"].
The word SPAM is actually a registered trademark of [http://www.hormel.com/ Hormel Foods Corporation]. Their [http://www.spam.com/ci/ci_in.htm trademark guidelines] say:
We do not object to use of this slang term to describe UCE, although we do object to the use of the word "spam" as a trademark and to the use of our product image in association with that term.
Also, if the term is to be used, it should be used in all lower-case letters to distinguish it from our trademark SPAM, which should be used with all uppercase letters. We hope that Spam with an uppercase letter S (as in SpamAssassin) is ok, too 
See also [http://en.wikipedia.org/wiki/Spam Wikipedia] for more infos on Spam, spam and SPAM.
Editorial notes:
This text needs some wordsmithing ["Ham" too]