if you've subscribed to a certain thread in the forum then it merely sends you an email each time a post is made and does nothing more, nothing less. Therefore I'm afraid the issue is with your email and not the forum (there is no issues with spam on the forum). Email web servers denote spam fr many, often innocent, reasons and each is often different, I personally still get the odd (non-forum I might add) email dumped in my spam folder for seemingly no apparent reason but its pretty rare these days.
I don't know whether other have experienced this but without me changing anything, all the Manchester bird forum alerts for Leighton moss have started going into my spam box as of about a couple of months ago. Has there been an increase in fake news