- Only IP with rDNS pointing to the same IP will be accepted, with FCrDNS validation;
- The rDNS must be in the domain of the MTA administrator, so third-party domains such as the generic rDNS of the data-center or ISP will not be accepted;
- The postmaster account must be configured for the domain registered on the rDNS and the account must be active and responding;
- There must be ways to identify the owner of the domain used at rDNS, for example by WHOIS system, and the information can not be hidden by privacy options;
- If you are using an IPv6 with SLAAC flag, you must keep port 25 opened to prove existence of an active SMTP service and
- Reputation of IP should be below 25% of negative points per total send volume.
Important: In the system SPFBL, none rDNS blocking rules are optional, and you also help report spam, as well as unpublished features such as global blacklist, spamvertised / URIBL block (URIs in the body of emails).
Tip: Consider using the SPFBL special rejection prefix to contain unwanted flow directly from your outgoing MTA: 5.7.1 SPFBL <message>
A script that is able to monitor this prefix and freeze email accounts can be a strong ally to prevent your IP from being listed. All negative points of our DNSBL are generated with rejections, at the SMTP layer, with this prefix. Minimizing the incidence of this prefix helps to have IPs always clean, with good reputation and a faster delist.