All emails to .co.za domains are failing for me

Fairly recently, I have started getting failure messages when sending to addresses ending in .co.za, using addresses that worked in the past. Example at the bottom. Facts:

Mailbox.org -> xxx@xxx.co.za FAILS

Mailbox.org -> xxx@xxx.com WORKS

Gmail - > xxx@xxx.co.za WORKS

The destination addresses are working, active addresses as I have used Gmail to successfully send them email. It's almost like there is a block between mailbox.org and *.co.za

I have tried with a mail client and with the Mailbox.org web interface, same result.

Any ideas? I really need to get this fixed.

Sending of the message failed.
An error occurred while sending mail. The mail server responded:
<admin@sfs.sygnia.co.za>: Recipient address rejected: Domain not found.
Please check the message recipient "admin@sfs.sygnia.co.za" and try again.