A blacklist is a list of IP addresses or domains that have been identified as spam emails previously. This list is created and gathered by independent spam organizations and most ISPs and companies would follow such list to filter incoming emails. There are also some companies that have their own internal blacklist system, which blocks emails that they think look spammy.
Getting your emails blocked or landing on the email blacklist has a direct impact on your email deliverability. Here are some best practices that you can follow to avoid your email from being blocked or blacklisted:
Reference: 15 Do’s and Don’ts to Avoid Having Your Email Blocked or Blacklistedby SparkPost