Since reputation systems became the primary method of spam filtering, the focus was more on IP address reputation and less on domain reputation. Now, IP address reputation is less important for inbox placement than domain reputation and message content.
With the advent of IPv6, it is possible for every electronic device in the world to have its own IP address. Because so many IP addresses are available, spammers can afford to waste them. ISPs cannot rely only on blocking IP addresses to protect their networks. IPv4 is still the default for email but that is changing, and spam filters are adapting.
Technology improvements progressed from simple IP reputation engines to ISPs and filter vendors that collect more metrics about email than ever before. Technologies like DKIM, SPF, and DMARC now make it possible to trust more data. Trusted data is used for reputation decisions and for spam filtering.
What to focus on
IP reputation is still critically important but is focused on the moment when the email is offered for delivery. When the mail is accepted, the rest of the reputational blend is used to determine whether the email is placed in the inbox or in the spam folder. The reputational blend includes domain reputation, content (including links and images), and customer interaction with the email.
Today, email marketers should focus on engagement at least as much as on IP address reputation. Authentication must be in place and correct. When that is done, focus on creating and maintaining a well-segmented database and an interested, engaged audience that interacts positively with marketing email. The more effective the email that is delivered, the better deliverability gets.