There are several reasons why an IP address might end up on the SBL: 1. Sending Unsolicited Emails: Sending emails to recipients who have not opted-in to receive them. 2. High Complaint Rates: Receiving a high number of spam complaints from recipients. 3. Spam Traps: Sending emails to addresses that are used specifically to identify spammers. 4. Poor List Management: Using outdated or purchased email lists that contain invalid or unengaged addresses.