That's been happening for months if not years - at least as long as I've been trying to use the email notifications. Every time I mark it as "not spam" but it still ends up in spam. I had to stop using email notifications on this forum consequently. I didn't bother reporting the issue because I'm tired of having to tell everyone about their misconfigured email systems.
this can be fixed by setting a filter specifically to catch the messages from OSdev.org and place them into the inbox
if you do this though, each message will appear with a header from gmail stating that this would have gone to the junk folder if it wasn't for that specific filter
marking not spam will never work because this site is black-listed