A shared IP address is used by multiple senders, offering cost-effectiveness, easier management, and scalability. However, deliverability is significantly influenced by the IP's reputation, which is tied to the sending practices of all users sharing it. Poor sending habits from one user can negatively affect deliverability for everyone. While IP reputation has become less critical since 2015, it still matters. New senders on shared IPs often require a 'warm-up' period, gradually increasing sending volume to build a positive reputation. Monitoring your sending reputation, using permission-based lists, sending relevant content, and authenticating emails are crucial for improving deliverability on a shared IP. Services like Amazon SES and SparkPost actively manage IP reputation to mitigate risks.