Best practices for email marketing in online gambling include segmenting the audience based on player behavior and preferences to deliver personalized content, such as tailored promotions and bonuses. It's crucial to create compelling and concise subject lines that attract attention while ensuring compliance with gaming regulations to avoid legal issues. Regularly cleaning and updating the email list helps maintain high engagement rates and reduces bounce rates. Additionally, employing A/B testing to evaluate the effectiveness of different email formats, calls to action, and timing is essential for continuous improvement.