1. Manual Data Entry Errors: Human error during data entry can result in incorrect email addresses or other essential information. 2. Outdated Information: Email addresses can become outdated if people change their jobs or personal email addresses. 3. Lack of Data Validation: Without proper data validation techniques, invalid or incorrect data can easily enter your system. 4. Duplicate Records: Duplicate email addresses in your list can lead to multiple emails being sent to the same person, causing annoyance and potential unsubscribes.