Hey,
If you're sending out letters to prospect for clients, generally, depending on your industry, you can just find potential clients via Google or any other means of getting their information. You could potentially get into some trouble with your ISP and/or any one of the stupid commissions for marketing and advertising if you start bombing people with SMS and email to a database you've bought and don't have an opt-in facility.
If you're talking about clients information that you have because you've sold them a service/product already, then your T&C's should cover any form of opt-in and your communication should contain an opt out.
There is also a large database of no contact people, but it's not really used/implemented correctly.
Also - something that doesn't really matter too much right now, but the POPI act, when/if it comes into effect will affect how you communicate with clients.