Website over IPv6 smtp.umd.edu has no AAAA record.
no
Mail servers (MX) have IPv6 None of the mail servers has an AAAA record.
saved.umd.edu ✗
no
Nameservers have IPv6 No nameserver has an AAAA record — an IPv6-only resolver can't reach this domain's DNS.
ns2.umd.edu ✗ns.ums.edu ✗ns1.umd.edu ✗
no
What's keeping smtp.umd.edu from an A
smtp.umd.edu is an alias of saved.umd.edu, which has no IPv6 smtp.umd.edu is a CNAME, so it can't also have its own AAAA record. Ask whoever runs saved.umd.edu 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 saved.umd.edu has no AAAA record, so mail from IPv6-only servers can't reach it directly. If a provider runs your mail, that's their setting; if you run it, add AAAA records for these hosts.
Nameservers without IPv6 ns2.umd.edu, ns.ums.edu, ns1.umd.edu 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