email.nova.cz is an alias of eu.mailgun.org, which has no IPv6 email.nova.cz is a CNAME, so it can't also have its own AAAA record. Ask whoever runs eu.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
Add IPv6 for www.email.nova.cz www.email.nova.cz has no AAAA record. Get www's IPv6 address from whoever hosts www and add it as an AAAA record. If the same server already serves both names (and its certificate covers www), you can reuse email.nova.cz's AAAA record(s) for www. How to add an AAAA record
Mail servers without IPv6 mxa.eu.mailgun.org, mxb.eu.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.
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