IP based reputation is a measure of the quality of the mail coming from a particular IP address. Because of how reputation data is collected and evaluated it is difficult for third parties to provide a reputation score for a particular IP address. The data has to be collected in real time, or as close to real time as possible. Reputation is also very specific to the source of the data. I have seen cases where a client has a high reputation at one ISP and a low reputation at another.
All this means is that there are a limited number of public sources of reputation data. Some ISPs provide ways that senders can check reputation at that ISP. But if a sender wants to check a broader reputation across multiple ISPs where can they go?
There are multiple public sources of data that I use to check reputation of client IP addresses.
Blocklists provide negative reputation data for IP addresses and domain names. There are a wide range of blocklists with differing listing criteria and different levels of trust in the industry. Generally the more widely used a list the more accurate and relevant it is. Generally I check the Spamhaus lists and URIBL/SURBL when investigating a client. I find these lists are good sources for discovering real issues or problems.
For an overall view into the reputation of an IP address, both positive and negative, I check with senderbase.org provided by Ironport and senderscore.org provided by ReturnPath.
All reputation sources have limitations. The primary limitation is they are only as good as their source data, and their source data is kept confidential. Another major limitation is reputation sources are only as good as the reputation of the maintainer. If the maintainer doesn’t behave with integrity then there is no reason for me to trust their data.
I use a number of criteria to evaluate reputation providers.

1. Personal history. I’ve been around this industry longer than most of the reputation sources. I’ve watched them develop, modify and adapt their policies over time.
2. How widely is the data used? This is particularly important when using blocklists. Lists that are widely used tend to have a more accurate listing policies as most large ISPs don’t want a lot of non-spam blocked.
3. How transparent is the provider? As I mentioned above there are good reasons for all reputation providers to keep their methods and data sources close to the vest. But that need for secrecy has to be balanced with enough transparency that their data and information can be verified by outside sources.
4. How do representatives handle themselves with listees and in public? Are they helpful or disdainful? Do they value the enduser and want the recipient to receive mail they want? The behaviour of employees and volunteers in public and with listees reflects the underlying values of the list and can be a major indicator of the value of the list.

Public sources of reputation data can be useful but not every source is created equally. When deciding what sources to listen to you need to evaluate the reputation of the source entity, too.

engagement • Reputation • Return Path Certified • senderscore