Sender score is the reputation rating that your IP address has raging from 0-100, which can be found for free on Senderscore.org. This Return Path service aggregates data from 60 million mailboxes at a variety of ISPs, spam filtering and security companies. The tool provides a comprehensive view of the direct measures of a sender's behavior and the impact those behaviors have on sender's brand and email deliverability. Return Path states that 83% of the time, sender reputation is the case of ISP filtering and not content.
Now typically a sender score of 74% or less will have issues reaching a users inbox. Your sender score can be negatively affected by spam complaints, high volume and frequency of mailings. Monitor all these points by cleansing your list regularly, keeping a consistent send pace and checking your external reputation often.