Just personal advice based on the many messages I receive.

At one point, you may need to contact a professional for the first time without prior knowledge between both of you. Maybe to ask about something, opinion, or advice. This is totally fine but let’s make this an efficient process for both sides, please send 1 message where you will:-

1-     Introduce yourself and where you work briefly.
2-     Give a brief idea about the topic you are working on, and why you think he is relevant.
3-     Ask your question.
4-     Finally, express gratitude.

A couple more things: -
1-     Don’t send this generic message, do some research about the guy first. To make sure he is the right one and to tailor your approach.
2-     Lower your expectations, nothing here is personal, maybe he even didn’t see the message at all.

Example:
Good Day Mr. Xxx
My name is Xxx I am working as xxx for Xxx company.
Currently, I am working on xxx and I believe that your expertise in xxx could be very helpful to me.
So from your experience, what I can do about xxx? How I can do xxx?
Thank you for your time.
Regards,
Xxx

Follow up, if you didn’t receive a reply on the 1st message, for example:-

Good Day Mr. Xxx
Your time is truly appreciated, and I am aware of its limitation, but I look forward to hearing from you.
Regards,
Xxx