Is It Acceptable to Contact the Professor of a Publicly Uploaded Lecture? A full graduate level course is publicly uploaded as a series of videos on Youtube.  Within the housekeeping section at the beginning of the course, the professor provides an email address as a means to contact him with questions.  No attempt is made to obscure this email address when the video is later uploaded.

Obviously, the professor's primary intent was for members of the class to have a way of contact, but would it be within the bounds of etiquette for a Youtube viewer following the course to email a question about the course material?  Suppose the question is of a sort that would not be well suited to ask in a Youtube comment.  Perhaps the question is long and includes images for clarification, but let's assume that the answer would not be proportionally long, that the professor could answer it fairly quickly and easily.  Would it be okay for a Youtube viewer not enrolled in the course (perhaps after the course had ended) to send such a question to the professor, maybe with the qualification, "Please do not feel compelled to take time out of your busy schedule to answer this, but I was watching your lectures on YT and was wondering..."?
Of course it is acceptable. But you also might not get a reply. The professor has to prioritise their actual students over someone who saw a Youtube video, and they are generally busy people that may not have time to respond to your question. However, they will not find it rude that you ask. You can find most professors' email addresses by simply Googling their name anyway.

To maximise the chance that you will get a reply I suggest writing your question in the most concise way possible (hint: not the way your question here is written) and in a way that clearly indicates that you've understood the basics in whatever field the lecture is about.