If you aren't familiar with what an email feedback loop is, or why you would want to set one up, please read Email Feedback Loops first, then come back here.
With the prerequisites out of the way, you're now ready to start sending in feedback loop applications.
In most cases, I recommend starting out by applying to the following feedback loops. The reason for this is that these ISPs have both a substantial customer base, and a reasonable application process. In other words, you get the most bang for your buck by starting with these:
After you've sent in the first tier application, I recommend also applying to the ISPs listed below. These are ISPs which in most cases won't have as many customers that you're sending to as the first tier ISPs. Still, if you're sending enough email to consider feedback loops, then at least some of them are probably going to recipients at these ISPs. These ISPs all have a straight forward feedback loop application process:
The following feedback loops have more complicated and/or restricted application processes than those in the first and second tier. I recommend evaluating these feedback loops on a case by case basis to determine if you qualify: