Check Out our own First Principles of Writing Effective Emails as a Starting Point for Crafting your own Best Content

Basics:

Make it about them, not about you
Stick to 4 sentences max (make it interesting and mobile-friendly)
Use conversational language
The smaller the ask, the easier it is for your recipients to answer (and more likely you'll get an answer)
The more personalized, the better - reference their name, company, industry, competitors, call out unique situations they're facing, reference a real (relevant) use case of your product by a similar company, use language THEY use, and offer specific solutions
- Leverage Apollo Snippets to automate your personalization
Follow-up, and follow-up your follow-ups

Subject line: Be informative, focus on clear value propositions, and make use of Apollo Snippets like {{first_name}} and {{company}} to be more personalized and targeted
Body: Keep it short and sweet, include clear value props, 4-6 sentences or about 800 characters or less in total, use Snippets to personalize your message

Subject Lines affect Open Rates
Email Bodies affect Response Rates
Test different version of your email body or subject line, but never both at the same time
Small changes in verbiage are more effective to test against each other than completely different content
Try to pinpoint what difference, whether subject line or opening sentence, is the cause for the change in results
Utilize the 'clone' function to quickly test small variations in your messaging