m.beeline.ru is an alias of beeline.ru, which has no IPv6 m.beeline.ru is a CNAME, so it can't also have its own AAAA record. Ask whoever runs beeline.ru 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 beekas-02.beeline.ru, fl-beekas-01.beeline.ru, beekas-01.beeline.ru, mxu.beeline.ru 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 ns3.beeline.ru, ns1.beeline.ru, ns2.beeline.ru, ns4.beeline.ru, ns1.beelinegprs.ru, ns2.beelinegprs.ru 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