PTR check
Check a mail server's reverse DNS: its PTR record, whether that name points back to the IP, and whether the HELO name fits.
How it works
A PTR record is reverse DNS: it turns an IP address back into a name, such as 192.0.2.10 → mail.example.com. Whoever controls the IP sets it, usually your hosting company, cloud provider or ISP, often under a setting called "reverse DNS".
Receivers such as Gmail and Outlook check it on every connection. A mail server without a PTR record, or with one whose name doesn't point back to the IP, looks like a hacked home computer, so its mail is rejected or sent to spam. Gmail requires every sender to have both.
"Forward-confirmed" (FCrDNS) means the PTR name has an A or AAAA record that leads back to the same IP. Names that contain the IP's digits, or words like dynamic, dsl or pool, look like home connections, and many filters treat them that way. That part is our own rule of thumb.
Your server's HELO name (the name it greets with) should be its PTR name, or at least resolve to its IP. Check its blocklist status with the blocklist check, or look up any record with DNS lookup.