Super, you'll either have to configure your email client to use SMTP authentication (and use your icon CONNECTION user/pass for the authentication) or use Sentech's SMTP server.
If you cannot send email using the authentication, then contact Icon and ask them why they've disabled it. I can't see Sentech only allowing traffic on port 25 to *their* SMTP servers (especially for fixed IP users), but test it anyway - telnet to Icon's SMTP server (I'll assume SMTP.ICON.CO.ZA) on port 25 : "telnet smtp.icon.co.za 25". If you can connect using telnet, it's the authentication screwing you around, if not then it's Sentech.
But really, it'd be easier (and probably fractionally quicker) to use Sentech's SMTP server. And unless Sentech's server has been blacklisted, there's no difference in what server you use to send mail.
BBJ