The problem *seems* to be fixed, for now. This is the explanation from support:
"Customers will experience a delay in the delivery of their
mail when sending via our smarthosts this includes
smtp.vstar.co.za (Vsat) customers, smtp.sentechsa.com (BBW
customers) and smtp.infosat.net (InfoSat Customers).
The cause for this is open mail relays on the BBW customers
machines due to software that is running on their machines
that are not configured properly enabling spammers to spam
via these hosts. They in turn use smtp.sentechsa.com to
relay the mail causing huge amounts of mail to be injected
in our outgoing mail queue."
Sounds a little suspect, wouldn't open smtp servers on customer machines usually just bypass smtp.sentechsa.com? Why relay through smtp.sentechsa.com if you run your own mail server? Perhaps their own smtp server(s) was/were open or hacked by spammers and they don't want to admit it, or perhaps customer machines were trojaned and used by spammers who for some reason relayed through smtp.sentechsa.com?