What's keeping email.agentprocessing.com from an A
email.agentprocessing.com is an alias of mailgun.org, which has no IPv6 email.agentprocessing.com is a CNAME, so it can't also have its own AAAA record. Ask whoever runs mailgun.org to enable IPv6 there — or replace the alias with the A and AAAA records they give you. Why some hosts don't offer IPv6
Mail servers without IPv6 mxa.mailgun.org, mxb.mailgun.org have no AAAA record, so mail from IPv6-only servers can't reach them directly. If a provider runs your mail, that's their setting; if you run it, add AAAA records for these hosts.
Nameservers without IPv6 ns1.agentprocessing.com, ns2.agentprocessing.com, ns3.agentprocessing.com, ns4.agentprocessing.com all have no AAAA record, so a resolver that only speaks IPv6 can't look your domain up. Most DNS providers offer IPv6 nameservers; ask yours, or move DNS to one that does.
Visitors on IPv6-only networks (many mobile carriers) can still reach it, but only through a translator their carrier runs (NAT64). The site itself has no IPv6 address — and you can't make one up from the IPv4. Why some hosts don't offer it
Copy the evidence— a citation-ready record of this measurement