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They are a email service so would have expected them to adhere to this standard. So yeah anecdotally for this transaction Protonmail have full FCrDNS servers. Įmail sent from IP address 185.70.43.25 - resolves to - which correspondingly resolves to IP 185.70.43.25 This all being said - I did a quick look at prontonmail and I dont see any issues around PTRs. If you are the only user or you feel you can stand your ground against your users then go ahead and ensure only FCrDNS are accepted. Admittedly the line needs to be drawn somewhere. and then you just get in arguments with users and other companies and its annoying. Personally I think only relaxed is necessary - that is bare minimum required by the RFC - as not all servers support FCrDNS - your users will complain that emails are not received if you implement FCrDNS. Other forms of validation should take over from that - SPF and more importantly DMARC. real mail servers please - so reverse lookup of IP address gives a domain name of some kind (and sometimes lazy names like 1-2-3-4.dingleberry are discarded) ie it cannot be some crazy home ISP address or script kiddy.

Relaxed meaning that at a minimum a valid email sending server MUST have a PTR record. Strict meaning that full FCrDNS is observed - reverse lookup of the IP address of connecting server gives a name (A record) and then forward lookup of that name gives the same IP address. Yes there are two levels of PTR validation a receiving server could do - Strict or 'Relaxed' essentially.

Protonmail address