High Bounce Rates: Sending emails to invalid addresses can increase bounce rates, signaling to ISPs that you don't maintain a clean email list. Spam Complaints: If recipients frequently mark your emails as spam, ISPs will view your emails as unwanted. Low Engagement Rates: Emails that are rarely opened or clicked can indicate that your content is not relevant, affecting your sender score. Blacklisting: If your IP address or domain gets blacklisted, your emails will be blocked by most ISPs. Inconsistent Sending Patterns: Sudden spikes in email volume can raise red flags, as can erratic sending times.