Generally if no one replies it's because no one knows the answer. It's doubtful that anyone here has experience with running Exim only to send mail on IPv6, because if exim would only send email on IPv6, then no other server not supporting IPv6 could receive mail from, or send email to, your server. Is that really what you want?
If your server only connects through IPv6, then the exim install will only work on IPv6. Exim by default uses the main server IP# as the outbound IP#.
However you cannot set the main server IP# as IPv6 because DirectAdmin's licensing system doesn't support IPv6, which means that DirectAdmin must use an IPv4 IP# as the main server IP#.
Perhaps
this guide will help you; have you tried using your IPv6 IP# instead of the IPv4 IP# listed in this guide?
Perhaps
this wiki (github.com) will have some useful information.
Otherwise you could probably create special rules in your firewall to refuse all inbound traffic on ports 25 and 443 of any IPv4 IP#, and to also route any outbound traffic from the server to any other server on port 25 or 443, to send email through one o fyour IPv6 ip#s.
Note these are just thoughts. Untried, untested, and only spent about fifteen minutes thinking about it.
Jeff