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.
Both the Courier and Dovecot mailservers can be used with cPanel to provide users with POP3 or IMAP access to their mail.
Many older cPanel installations will be running Courier. Interestingly Courier's default config already has IPv6 enabled by default. IPv4 addresses in log files are written in IPv6 compatible format by prepeding them with "::ffff:".
In order to connect to your mail server via IPv6 you can open your favourite mail client (Thunderbird is known to work) and set the server name to your servers IPv6 address. View the log files at "/var/log/maillog" to see your client login via IPv6.
Next step would be to add a DNS entry for your mail server. If you are the only user on your mail domain then you can add the IPv6 address of your server as a AAAA record for mail.yourdomain.co.za. If you have many other users on your domain then you may want to rather add the AAAA for mail6.yourdomain.co.za. Users with broken IPv6 connectivity may experience higher latency or inability to connect if the AAAA is applied directly to the 'mail' hostname.
Dovecot requires one change to enable IPv6: the listen directive needs to be adjusted to include both IPv4 and IPv6. Access the dovecot config template and add the line "listen = *, [::]" and rebuild the config. This should make dovecot IPv6 capable but this has not been tested.
I sometimes have mixed feelings for spammers. While I mostly despise them for the time and money they waste in my life - I can't help being a little impressed every now and again.
There has thus far not been any noticeable email spam hitting my mail server on its IPv6 address. This is a relief since the availability of IPv6 RBLs and other spam fighting filters is currently limited.
I was however privileged to receive my first IPv6 comment spam on this blog recently. This means that one of the botnet herders has either included IPv6 support in their code or they are writing good enough code that it is IP version agnostic. Is this a good thing...?
I had hoped that the IPv6 Internet would be free of the evils of the IPv4 Internet. As long as it stays niche there will be no reason for the worst netizens to show their faces on this side of the great IP divide. Then of course we would not be achieving our end goal of moving everyone (including the spammers) to IPv6.