Exim has a mature IPv6 implementation and it is likely to be the first thing to start using the IPv6 connection you setup on your cPanel server. Most other services will only be used when you add a AAAA record to the relevant DNS zone.
Since SMTP also makes outbound connections it will immediately attempt to use IPv6 when attempting to communicate with other IPv6 capable mail servers. For this reason care should be taken when enabling IPv6 as it can potentially cause mail delivery problems. Ensure that at the very least the rDNS entries for your server's IP are correctly configured.
My biggest concern with mail delivery over IPv6 is that many antispam solutions do not properly understand an IPv6 address. Depending on how well a particular server is setup it may accept IPv6 mail or potentially discard it randomly and unpredictably. It is important to be reviewing you logs to identify mails that are being delivered (or not) over IPv6.
Configuring inbound mail also requires some care to ensure that your spam filters do not reject IPv6 email. It is currently fairly safe to apply an accept all rule matching all IPv6 mail but this is not going to continue.
cPanel normally uses the same A record for your website and your mail. For this reason I suggest that you don't just add a AAAA to your sites main hostname since this will cause slowdowns on your website if your IPv6 setup is not perfect. The solution to this is to add a new A record for specifically for mail: mx.yourdomain.co.za. Then add an AAAA for the same hostname and adjust your MX records to point to your new hostname.
You then want to get someone to send you mail via IPv6. The easiest way to do this is to join a mailing list that runs on IPv6 enabled servers.