How can I get feedback for my work in math if I'm not in academia? I will soon finish my undergraduate degree in math, and I want to study mathematics as a hobby after I graduate.

Online, I see a general trend of people saying


  You can't learn real math without collaboration and feedback. Go to graduate school!


But I don't want to go to graduate school, and I have found that I learn better through self-study than by taking notes in class. That being said, it always does help to get my questions answered by professors, and collaboration with other students is sometimes helpful as well.

Question:  Is it possible to have this kind of mathematical community without actually being in academia?

What I fear is studying for years by myself obsessively on a subject only to find that I had a horrible misconception the whole time that could have been avoided if only a professor had looked at my work.
Nowadays, you can get feedback and answers to questions from places like MathStackExchange, and MathOverflow. Most of the responses are (in my observation) either entirely competent or quickly corrected by others. Good quality control. No, that wouldn't be the same as having a good advisor (ideally a world-class scholar on your subject) that you talk to in person every week. Still, these sites can give guidance.

Also, quite a few of the more-senior people who like to contribute to such sites may be amenable to direct email contact, etc.

(Grad school is not about "taking notes in lectures" per se, but is about having access to the informal observations of very good mathematicians, and being able to ask them questions immediately, in the moment.)