In permission-based email marketing, the Inbox Placement Rate (IPR) is a deliverability benchmark that is used to determine what percentage of emails arrive at the recipients' inbox. It is a key metric that email marketers focus on nowadays.
Every ISP keeps a record of your reputation score. Every time you send mass emails, your reputation score is recalculated based on the complaint rates, number of expired email addresses and unknown accounts in your email list. Once your reputation score drops below the threshold, your account or IP address will be suspended or blacklisted.
Therefore, after each email marketing campaign, remember to clean up your email list by examining the open rate, click rate and website activity of each email address.
Reputation, once ruined, is hard to rebuild!