A grade should be an A with TLS enabled. When I've seen B+ or lower I see companies like Microsoft, Google, etc. marking them as spam due to the fact that they have older insecure protocols. I don't necessarily agree with that but that is a factor.
Also, do you have a reverse DNS setup? That could throw you in. They try to make lives better by stopping spammers. An option could be contacting them and asking WHY so you can adjust. It could be something simple like the domain is too new, there used to be a spammer on your IP address (or still is on your class C), etc.