The rule: Be specific about what you need from the recipient.
There is usually a clear intent behind an email. You may want the recipient to answer a question, provide information, take an action, forward your email or even just be aware of the information you shared. In all these cases, a clear "call to action" helps the recipient better understand what you're expecting them to do.
So, avoid open ended questions. Instead of “thoughts?” ask specific questions like "What would you like us to do about X...." or "Do you agree with my opinion that Y..."
Just need the recipient to be aware of something? Add "FYI" or "I thought you should know about this.” The best option? Clearly state "no action needed.”
Key takeaway: Nobody is a mind reader (probably). Be just as detailed as to the type of response or action you need as you are with the information you’re sending.