Monitoring blacklists involves checking if your email server’s IP address has been flagged by one of the numerous blacklist services such as Spamhaus, SORBS, or Barracuda. Typically, these services use a variety of criteria to judge if an IP should be blacklisted, including spam reports, sending volume spikes, and email content issues. According to Return Path's "2019 Deliverability Benchmark Report," more than 20% of emails worldwide fail to reach the inbox, with blacklisting being a contributing factor.