FCrDNS stands for Forward Confirmed Reverse DNS and it is used to show relationship between an IP address and domain name. To explain this let's look at the following examples:

Forward DNS:

example.com (domain) → 1.2.3.4 (IP)

Reverse DNS:

1.2.3.4 (IP) → example.com (domain)

FCrDNS means that the above Forward and Reverse DNS must match. In the above case they do so FCrDNS is confirmed. Although a weak form of authentication it is a good practice to have FCrDNS as it is used among many mail providers as an anti-spam method which in turn can help with email deliverability and placement. 

See also: Reverse DNS

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