I'm not sure if we can avoid that in a simple way. Our newsletter providers are also working with other companies and sending their emails, so probably some of them people are marking as a spam. The only way to not getting our newsletter into the spam folder is marking our emails as "This is not a SPAM". With time email provider's mechanism will learn that it should stop blocking emails or sending them to spam. I don't know if there is a better solution for this problem.