Is it acceptable to ask a professor for advice regarding a subject not taught in, but related to their class? I'm having a hard time understanding some concepts not explicitly taught in a course I took recently, but quite related nonetheless. I'm wondering if it's okay to e-mail the professor who taught this course and ask them for advice regarding this matter, considering:


I am no longer a student enrolled in one of their courses,
The subject matter I want to ask about was not part of that course's curriculum, even if it does come (very?) close.


Knowing this I've been wondering whether or not it's reasonable for me to approach this teacher and ask for help.

I've tried my best to research this site for simliar questions, but I've come up with little. The best I could find is only tangentially related at best, since it's not a homework matter.
Is it acceptable to ask a professor for [...]?


Yes, the worst it can happen is that it doesn't answer.

But: be polite, short and to the point (if you have a long question, ask for an appointment), and don't expect an answer in two hours.