Sending mail to certain recipients, results in a "delivery error report" generated by smtp.wbs.co.za which then states the reason from the remote side which is "smtp.wbs.co.za listed in sorbs.net"
> The following message to <*************************.com> was
> undeliverable.
> The reason for the problem:
> 5.1.0 - Unknown address error 553-'Rule imposed as
> smtp.wbs.co.za is blacklisted on Sorbs (see www.sorbs.net)'