High Bounce Rates: Sending emails to invalid addresses can increase bounce rates, signaling to ISPs that you have poor email list hygiene. Spam Complaints: If recipients frequently mark your emails as spam, it indicates to ISPs that your emails may be unwanted or unsolicited. Low Engagement Rates: Low open and click-through rates can suggest that your emails are not relevant or interesting to your audience. Blacklisting: Your IP address or domain can be blacklisted if you're flagged for sending spam or malicious content. Poor List Management: Using purchased or outdated email lists can lead to high bounce and complaint rates.