A reverse DNS check should help you decide what to investigate next. The updated Reverse DNS Lookup shows the reverse query name, PTR answers, TTL values and forward-confirmation result together.
Missing and unavailable are different
A resolver can answer successfully without returning a PTR record. It can also time out or refuse a query. The page now distinguishes those cases. A failed lookup leaves the answer unknown; it does not establish that the IP has no reverse record.
The same applies to forward confirmation. If a PTR hostname resolves back to the original IP, it is confirmed. If the forward request fails, the page says that confirmation is unavailable instead of presenting a definite mismatch.
Less waiting between queries
The backend groups independent DNS requests into batches and briefly caches reverse results. This reduces serial waiting when a hostname has several addresses. It still applies limits, so a single lookup cannot expand into an unlimited chain of DNS questions.
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You can copy a link with the IP or hostname already filled in. Opening that link runs a lookup; it does not preserve the old answer as evidence. DNS may have changed by the time someone else opens it.
For mail setup, read our reverse DNS guide. If a PTR needs changing, start with the provider that manages the IP allocation. The ordinary DNS panel for your domain may not control the reverse zone.