Question
Can I use Exim with IPv6 priority or IPv6 only?
Answer
At this time there is no method to prioritize or only use IPv6 for Exim. cPanel intentionally patches Exim to prefer IPv4 over IPv6 when IPv4 is available.
We have the following feature request which covers IPv6 support for various services:
https://features.cpanel.net/topic/ipv6-support-for-the-rest-of-the-services