10 Simple Practices to Improve Inbox Delivery
No matter what email delivery service or SMTP relay you are using, some of your messages can end up in the spam folder. Internet service providers use different spam filtering techniques (which are sometimes pretty odd) to protect their users from spam.
So, you will absolutely have some emails filtered as spam or junk mail from time to time when sending your email campaigns.
There are tools like this free email spam checker that you can use to test your spam score, Inbox rate and Spam rate prior sending a real email campaign. The results may surprise you.