- Litmus: Litmus is a comprehensive tool that offers email previews across various clients and devices, spam testing, and analytics. - Mailtrap: Mailtrap is great for testing SMTP servers and previewing emails before they go live. - Mail-Tester: This is a spam testing tool that gives you a detailed report on how to improve your emailâs deliverability. - Optimizely: Known for its robust A/B testing capabilities, Optimizely can help you optimize your email campaigns.