Forward-confirmed reverse DNS: does the IP's PTR hostname resolve back to the same IP? Mail servers check this.
Checked live via DNS-over-HTTPS. Nothing is stored.
Two lookups that must agree: the IP's PTR record gives a hostname, and that hostname's A/AAAA record must list the same IP. Many mail servers reject or penalize senders that fail this check — it's one of the cheapest spam filters in existence.
The classic cause: the PTR points to a hostname whose A record points somewhere else (often a generic ISP hostname). Fix it by setting the PTR to your mail server's hostname and making sure that hostname's A record points at the IP.
For a mail server: bad. Gmail and others treat missing reverse DNS as a strong spam signal. For web servers or home connections it usually doesn't matter. Your hosting provider sets the PTR — it's not a DNS record you can publish yourself.
Yes — it's resolved to its public IPs first, then each address gets the full reverse→forward check.
No — they complement each other. FCrDNS is about your DNS hygiene; the blacklist check is about your IP's reputation. Run both before blaming content for delivery problems.