With the introduction of the "Electronic Communications and Transactions Act, 2002" unsolicted emails now have a legal definition and must meet certain requirements.

Here is the specific text of the Act relevant to spam:

Unsolicited goods, services or communications

45.(1) Any person who sends unsolicited commercial communications to consumers, must provide the consumer

(a) with the option to cancel his or her subscription to the mailing list of that person; and

(b) with the identifying particulars of the source from which that person obtained the consumer'spersonal information, on request of the consumer.

(2) No agreement is concluded where a consumer has failed to respond to an unsolicited communication.

(3) Any person who fails to comply with or contravenes subsection (1) is guilty of an offence and liable, on conviction, to the penalties prescribed in section 89(1).

(4) Any person who sends unsolicited commercial communications to a person who has advised the sender that such communications are unwelcome, is guilty of an offence and liable, on conviction, to the penalties prescribed in section 89(1).

You can also read the following legal opinion on the matter:
Legal View: The Law vs the Scrouge of Spam (Lance Michalson, 2003)

Source: ispa.org.za