Okay... I contacted Daniel, who as per my previous post was listed as Admin and members told me was the person to contact at the time.
Sparkpost have indeed now changed their pricing for new customers, though it was available at the time of writing. Our community would also have a problem with 500 emails a month as we're a similar size to this one, so I'm familiar with the scale.
I don't know what your stack is, but there are steps you can take to improve your community's trust level with major mail providers - However it could end up being more costly and complex to implement than integrating a third-party service for email, depending on what your platform is.