@noxy you will need to own a domain so you can do that and africanhuts.co.za is not a valid domain name. africanhuts.co would be a valid domain name.
Like for example for your domain sive.host you have MX records mail exchanger = 0 mail.sive.host. you should do the same for your new domain. Add mx records to public dns for domain you want that point to your proxmox mail gateway server public ip that has port 25 port forwarded.
https://en.wikipedia.org/wiki/MX_record
Basically you buy a domain you want add basic domain records so that domain is responding and mx records that points to public ip address of your proxmox mail gateway and once the dns records are propagated you can try to send a test mail and you should see it in tracking center.
Let me know if that helps...